Even though you don’t know it, you probably have an app (or several), music or something else that transferred over from your old device that was originally downloaded or purchased using your old account. If you delete it, it should stop prompting. If you can’t figure out what it is, setting up as a new phone should work.

Upfront, it’s possible to make it BETTER, but not solve it. I forgot how though.

I had this as well, I think the app it was calling finally got removed from the App Store lacking dev support. I never solved it, but the prompts eventually stopped. I don’t remember how, though. I think you can look at app install history and might be able to find what was bought by that account and maybe rebuy that app with your current account?