"Apple cash requires the same Apple ID..." I have complied with this and it still doesn't work!

I spoke with Apple Pay support. They instructed me to attempt to send Apple Cash to another recipient and I was able to start to do that without the error message. So their solution was to delete the iMessage threads to the originally intended recipient and try again. And that worked!

No problem Eric, and thanks for your interest. What's curious is that the solution had nothing to do with Apple IDs as the error message suggested.

Yes. It's the only Apple ID I have.

And I've been using it for a long time without any problems until now.