Same problem here. I have spent 5 hrs trouble shooting between the apple pay team and my bank's digital wallet team. I cannot load money from my bank to my Apple Cash account. The transaction goes through on the bank end, and then apple declines it and refunds the money to my bank account. Also I had 1 transaction go through this morning. Then afterwards all transactions not accepted by apple. The apple tech finally said it looks like a problem on Apples side; their "back end" and I should wait a few days and then try again. When I stated this wasn't acceptable they transferred me to a supervisor who said Apple can decline any transaction they want and they don't have to give you a reason.
I was greatly disappointed with the attitude given by the last person (supervisor). Totally not the typical Apple support!
Interesting note - when I called my bank back to let the tech know how it was going he said he had had 3 other calls today with the same issue with Apple Pay. Fascinating.....