It has to be a current or business account for Managed Payments, and not a savings account.
There are current issues on the app so if using this, switch if you can to a laptop / PC and try Chrome that seems to work best then enter details here:
https://ppcapp.ebay.co.uk/myppc/wallet/list
If there are still issues contact eBay Customer Services. They have access to your account and may see where the issue lies.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open between 8 a.m. to 10 p.m. on weekdays.
9 a.m. to 6 p.m. on weekends.
I recommend contacting CS very first thing in the morning, when the lines first open, as there's more chance of Dublin answering.
@pithea