I have two different Apple ID's but suddenly I can't switch between the two, Why?

I have two accounts with two different Apple ID's - a legacy thing that goes back years but I can't merge the two. No big deal - for several years now, I've favoured one heavily over the other while I still can, on rare occasions, access older music or movies by switching to the less used ID.

Just a few minutes ago, I tried to sign up for Apple One. The sign-in window that came up had my older, less used Apple ID in the sign in window. I deleted it and replaced it with the ID that I regularly use. After I put in the password for this account, the computer said "NO" and threw me back to the other account that I don't want to use. How do I tell Apple that the ID I've been using for Apple Music, this forum and most things currently IS the ID I want use and not the other one?

BTW, a couple of hours earlier, I updated my 2020 MacBookPro to Big Sur 11.5