Solved: Apple Pay "Payment Not Completed" with red exclamation mark

Issue: I attempted to use Apple Cash to pay / transfer on my device. For example, buying from the App Store. Not point of sale in a physical store and not a website-based purchase.

Symptoms: The prompt to touch the side button twice appeared, FaceTime attempted to authenticate and then I would see a red exclamation mark and the payment would not complete.

Others have stated various fixes such as a FaceTime reset, card details that weren't up to date (like address) etc. None of this applied to me. My details had not changed and I was using a positive balance in my Apple Cash, not a credit card. So I know funding was available. And finally, there was no outage reported by Apple regarding Apple Pay. Resetting my phone did not work either.

Here's what worked for me:

I wanted to see if Apple Cash itself was the issue.

1. To do this I attempted a trivial bank transfer to move $1 from my Apple Cash back into my bank. Again, this only just to see if Apple Cash was "broken" somehow.
2. After the very last step before initiating the transfer, a window appeared explaining that I needed to agree to a new Terms of Service agreement regarding transfers*.
3. I agreed to the new Terms.
4. I didn't even finish the $1 transfer test because I figured agreeing to the new terms was the issue: and it was. The moment after I accepted the new Terms of Service, I went back to where my original payment issue was occurring and the Apple Pay issue was resolved.

Hopefully this helps some of you out there! :-)

*I believe these were not terms to use Apple Pay overall; rather they were new terms that governed transfers specifically. This would explain why I could use Apple Pay at a register or on a website but trying to make a digital transfer wouldn't work.