Enum list not letting me add a value?

[1billyfw]

Hi guys - 

I have two relevant tables: DB_Transactions and DB_Rules. The data sources are Google Sheets. Within DB_Transactions is a non-identifying column, PayeeName. Within DB_Rules is a column "NewPayeeName". 

In a form for DB_Rules, I would like NewPayeeName to be a dropdown with the values from DB_Transactions[PayeeName], with the ability to put in a value that is new/not in DB_Transactions[PayeeName]. 

This is the configuration of the field: 

But in the Form, it won't let me add a new value: 

When I click "Done" I return to the form but the field is blank. 

Any help would be greatly appreciated. Thanks a lot!

Billy