I'm getting the error ``This Tax Agency is used in the existing registration and cannot be deleted`` even after deleting all transactions for the vendor

I’d be looking to see if it’s a linked tax vendor to a tax code. (I’m not online hopefully that’s enough to get you looking in the right place)

I agree - Take a look in Setup > Accounting > Tax Codes and see if any of the tax codes has that Tax Agency selected.

I'll take a look at that, thank you!!!