I had this exact problem. I also spoke to Apple support and was escalated through their ranks until eventually it passed to their engineers to investigate. I heard back from them yesterday and it turns out I was entering the incorrect routing number for my bank. I was using a routing number that was on a deposit slip from an old checkbook, but it turns out that my bank (Chase) uses an internal routing number scheme for deposits. So the routing number I was attempting to use was invalid.
I never realized that the routing numbers could/would be different and the error message made me think the problem was on Apples end, not mine. Also, the Apple Cash card allowed me to add this incorrect bank account info without issue. This reinforced my thought that I was doing nothing wrong.
Once I found the correct routing number, I was able to successfully add my bank account info to Apple credit card. ( I also removed the incorrect bank info that Apple Cash allowed me to add. )
Hope this helps someone.