I don’t use GarageSale often as I do not sell on eBay often, but I did not have this error a year ago. I get under Preflight:
“A payment method has to be set up so you can continue to sell on eBay.”
Well I have PayPal and credit cards attached to my eBay so I cannot think what the problem could be. I have seen other people mention this sort of thing in 2016 and 2018 on this forum, but I did not find solutions there.