I would suggest creating methods for all commands just like the help and calling them after the elsif for example:

elsif (@@user_command.include? 'add')
+      PM.add(*params)


def self.add(description, priority, pid)
 #add 2 take out the trash 1
       puts "#{@@user_command}".colorize(:yellow)
       pid = @@user_command.slice(4).to_i
       priority = @@user_command.slice(@@user_command.length - 1).to_i
       description = @@user_command.slice(6..@@user_command.length - 2)
       @@project_list.add_task_to_project(description, priority, pid)
       chosen_project =  @@project_list.projects.find{|project| project.id == pid}
 
       puts
       puts "task was added to the #{chosen_project.project_name} project".colorize(:yellow)
       puts
end