The error message is telling you that there is already a button on the screen with the name to register as ‘PrimaryADD’. By default when a button is added and it is assigned as a new button name to register gets ‘PrimaryAdd’ You will need to look at all of the buttons that are for adding new records on that screen. If you want to check via sql prior to searching the screen you could use a simple script to confirm that this is the issue.
select * from cs_item where screen_name = ‘your screen name’ and control_type = ‘Button’ and register_name = ‘PrimaryAdd’
if you get back more than one line then you know that is where the problem is.