This issue occurs when there are related records with a restrict-delete association that blocks record deletion.
To fix this issue, identify the relevant entities with such relationships, and then delete the records of those entities related to the record you are now trying to delete.
Go to Settings > Customizations.
Select Customize the system.
Locate the entity with records you want to delete (for example, Accounts).
Go to 1: N Relationships for the entity.
Find any relationships where the Type of Behavior is Referential, Restrict Delete.
Take note of the related entity for each of those relationships; these are the type of records that may be restricting your ability to delete the record.