Измени этот метод 
public void TransferData()
{
    using (var sqliteConnection = new SQLiteConnection(connectionString))
    {
        sqliteConnection.Open();
        string selectQuery = "SELECT * FROM project"; // Замените на ваш SQL-запрос для извлечения данных

        using (var sqliteCommand = new SQLiteCommand(selectQuery, sqliteConnection))
        using (var reader = sqliteCommand.ExecuteReader())
        {
            // Объявляем словари для хранения идентификаторов
            var employeeIds = new Dictionary<string, int>(); // идентификаторы сотрудников
            var projectIds = new Dictionary<string, int>(); // идентификаторы проектов
            var teamIds = new Dictionary<string, int>(); // идентификаторы команд

            // Коллекции для хранения данных для вставки
            var employeesToInsert = new List<(string FirstName, string LastName, string Email)>();
            var projectsToInsert = new List<(string ProjectName, DateTime StartDate, DateTime EndDate, string Status)>();
            var tasksToInsert = new List<(string TaskName, string AssignedToEmail, string ProjectName, string Status, DateTime DueDate)>();
            var teamsToInsert = new List<(string TeamName, string TeamLeaderEmail)>();

            // Считываем данные из SQLite
            while (reader.Read())
            {
                // Извлечение данных из SQLite
                string firstName = reader["first_name"]?.ToString();
                string lastName = reader["last_name"]?.ToString();
                string email = reader["email"]?.ToString();
                string projectName = reader["project_name"]?.ToString();
                DateTime projectStartDate = DateTime.Parse(reader["project_start_date"]?.ToString());
                DateTime projectEndDate = DateTime.Parse(reader["project_end_date"]?.ToString());
                string projectStatus = reader["project_status"]?.ToString();
                string taskName = reader["task_name"]?.ToString();
                string taskStatus = reader["task_status"]?.ToString();
                DateTime taskDueDate = DateTime.Parse(reader["task_due_date"]?.ToString());
                string teamName = reader["team_name"]?.ToString();
                string teamLeader = reader["team_leader"]?.ToString();

                // Собираем данные для вставки
                employeesToInsert.Add((firstName, lastName, email));
                projectsToInsert.Add((projectName, projectStartDate, projectEndDate, projectStatus));
                tasksToInsert.Add((taskName, email, projectName, taskStatus, taskDueDate)); // Email исполнителя
                teamsToInsert.Add((teamName, teamLeader.Split(new[] { ", email: " }, StringSplitOptions.None)[1].Trim())); // Email лидера команды
            }

            // Соединение с PostgreSQL
            using (var postgresConnection = new NpgsqlConnection(PostgresConnectionString))
            {
                postgresConnection.Open();

                // 1. Вставка сотрудников
                foreach (var (FirstName, LastName, Email) in employeesToInsert)
                {
                    using (var employeeCommand = new NpgsqlCommand(@"
                INSERT INTO employee (first_name, last_name, email) 
                VALUES (@first_name, @last_name, @email) 
                ON CONFLICT (email) DO NOTHING RETURNING employee_id", postgresConnection))
                    {
                        employeeCommand.Parameters.AddWithValue("@first_name", FirstName);
                        employeeCommand.Parameters.AddWithValue("@last_name", LastName);
                        employeeCommand.Parameters.AddWithValue("@email", Email);

                        var employeeId = employeeCommand.ExecuteScalar();
                        if (employeeId != null) // Проверка на null
                        {
                            employeeIds[Email] = (int)employeeId; // Сохраняем ID
                        }
                        else
                        {
                            using (var checkEmailCommand = new NpgsqlCommand("SELECT employee_id FROM employee WHERE email = @Email", postgresConnection))
                            {
                                checkEmailCommand.Parameters.AddWithValue("@Email", Email);
                                employeeIds[Email] = (int)checkEmailCommand.ExecuteScalar();
                            }
                        }
                    }
                }

                // 2. Вставка проектов
                foreach (var (ProjectName, StartDate, EndDate, Status) in projectsToInsert)
                {
                    using (var projectCommand = new NpgsqlCommand(@"
                INSERT INTO project (project_name, start_date, end_date, status) 
                VALUES (@project_name, @start_date, @end_date, @status) 
                ON CONFLICT (project_name, status) DO NOTHING RETURNING project_id", postgresConnection))
                    {
                        projectCommand.Parameters.AddWithValue("@project_name", ProjectName);
                        projectCommand.Parameters.AddWithValue("@start_date", StartDate);
                        projectCommand.Parameters.AddWithValue("@end_date", EndDate);
                        projectCommand.Parameters.AddWithValue("@status", Status);

                        var projectId = projectCommand.ExecuteScalar();
                        if (projectId != null) // Проверка на null
                        {
                            projectIds[ProjectName] = (int)projectId; // Сохраняем ID
                        }
                        else
                        {
                            using (var checkEmailCommand = new NpgsqlCommand("SELECT project_id FROM project WHERE project_name = @Name", postgresConnection))
                            {
                                checkEmailCommand.Parameters.AddWithValue("@Name", ProjectName);
                                projectIds[ProjectName] = (int)checkEmailCommand.ExecuteScalar();
                            }
                        }
                    }
                }

                // 3. Вставка задач
                foreach (var (TaskName, AssignedToEmail, Projectname, Status, DueDate) in tasksToInsert)
                {
                    int assignedToId = employeeIds.TryGetValue(AssignedToEmail, out int empId) ? empId : -1; // ID исполнителя

                    using (var taskCommand = new NpgsqlCommand(@"
                INSERT INTO task (task_name, assigned_to, project_id, status, due_date) 
                VALUES (@task_name, @assigned_to, @project_id, @status, @due_date)
                ON CONFLICT (task_name, assigned_to, project_id, status) DO NOTHING", postgresConnection))
                    {
                        taskCommand.Parameters.AddWithValue("@task_name", TaskName);
                        taskCommand.Parameters.AddWithValue("@assigned_to", assignedToId);
                        taskCommand.Parameters.AddWithValue("@project_id", projectIds[Projectname]);
                        taskCommand.Parameters.AddWithValue("@status", Status);
                        taskCommand.Parameters.AddWithValue("@due_date", DueDate);
                        taskCommand.ExecuteNonQuery();
                    }
                }

                // 4. Вставка команд
                foreach (var (TeamName, LeaderEmail) in teamsToInsert)
                {
                    int leadId = employeeIds.TryGetValue(LeaderEmail, out int id) ? id : -1; // ID лидера

                    using (var teamCommand = new NpgsqlCommand(@"
                INSERT INTO team (team_name, lead_employee_id) 
                VALUES (@team_name, @lead_employee_id) 
                ON CONFLICT (team_name, lead_employee_id) DO NOTHING RETURNING team_id", postgresConnection))
                    {
                        teamCommand.Parameters.AddWithValue("@team_name", TeamName);
                        teamCommand.Parameters.AddWithValue("@lead_employee_id", leadId);

                        var teamId = teamCommand.ExecuteScalar();
                        if (teamId != null) // Проверка на null
                        {
                            teamIds[TeamName] = (int)teamId; // Сохраняем ID
                        }
                        else
                        {
                            using (var checkEmailCommand = new NpgsqlCommand("SELECT team_id FROM team WHERE team_name = @Name", postgresConnection))
                            {
                                checkEmailCommand.Parameters.AddWithValue("@Name", TeamName);
                                projectIds[TeamName] = (int)checkEmailCommand.ExecuteScalar();
                            }
                        }
                    }
                }

                // 5. Вставка в таблицу project_team
                foreach (var projectEntry in projectIds)
                {
                    foreach (var teamEntry in teamIds)
                    {
                        using (var projectTeamCommand = new NpgsqlCommand(@"
                    INSERT INTO project_team (project_id, team_id) 
                    VALUES (@project_id, @team_id) 
                    ON CONFLICT (project_id, team_id) DO NOTHING", postgresConnection))
                        {
                            projectTeamCommand.Parameters.AddWithValue("@project_id", projectEntry.Value);
                            projectTeamCommand.Parameters.AddWithValue("@team_id", teamEntry.Value);
                            projectTeamCommand.ExecuteNonQuery();
                        }
                    }
                }
            }
        }
    }
}

так чтобы не было ошибки: 23503: INSERT или UPDATE в таблице "team" нарушает ограничение внешнего ключа "team_lead_employee_id_fkey"
18:16:05:296	
18:16:05:296	DETAIL: Detail redacted as it may contain sensitive data. Specify 'Include Error Detail' in the connection string to include this information.