The error message is telling you that either Status or Work_Item_Status__c is a text field. Since Status is a standard picklist, we know it is in fact ``Work_Item_Status__c` you are handling incorrectly. The correct way to check text equivalence in a formula is:

Work_Item_Status__c = "Done"

The error message is telling you that either Status or Work_Item_Status__c is a text field. Since Status is a standard picklist, we know it is in fact Work_Item_Status__c you are handling incorrectly. The correct way to check text equivalence in a formula is:

Work_Item_Status__c = "Done"