Unfortunately I have the same problem. The problem remains even if I remove all clubs from the UEFA competitions. I've been using the same data set since the FM20. I have always transferred this to the new version and it runs without any problems. With the current file I can also play the FM24 without any problems. This is a changed Wales league system. With extended, own rules. However, no changes are now possible. As of today I can no longer verify leagues because I always get this
"UEFA Euro League, Chelsea has no squad selected"
Error in between works. I haven't changed any other rules except Wales's. The clubs vary depending on the run. Sometimes it's Chelsea, sometimes it's someone else.
I just can't find the solution. The error is also not related to changes to the league system/rules. I have only adjusted a few player details since the last verification.
Edit: I have solved the problem for myself. I added Chelsea to the Champions League and the error no longer appeared. Not even with Juve or Inter.