The situation can occur to your "ONE" (Parent table) table if you have the following setting "Cannot delete if related many" selected for your relation. In order to resolve the issue:
-Open the inspector window for your relation. (Double click on the relation line)
-Click on the control tab.
- Select "Leave related many Intact" in the deletion control section.
-Click apply.
-Delete relation.
-Uncheck the unique and can't modify properties of the field in the one table that you are relating with your many table.
-Draw the relation from the many table to the one table. View the related field in the one table, the unique and can't modify attribute should not be automatically enabled.