Try removing the unique constraint on the attribute and run again?
The answer of Adrian is not the right answer. You could get these kind of errors when you upgraded the Mx6 database to Mx7. Because in Mx6 these checks where not done in the database but in the modeler. But in Mx7 you can still get those errors also.
If you make changes to your validation rules that are incompatible with existing data, you will need to migrate your data before you can apply those validation rules. You can do this in the on app startup microflow.