What you describe sounds like an error message from eBay (not from GarageSale). Did you actually already start a listing with the eBay account in question or is this a brand new (seller) account?
If not done already, please also refresh your eBay access tokens in the GarageSale preferences > Accounts.
I had mayor issues when trying to list something after my account was switched to managed payment on eBay.de.
The only workaround I found was to start to list an item on the eBay website, which at some point presents you with a form to fill-in your bank details, even though I already filled those in multiple times.