Ideas
I managed to fix this, but on PC. I had to delete all the squad files and personal settings from the file in the appdata: C:\Users\[YourUsername]\AppData\Local\EA Sports FC 26 (make sure you have hidden files set to visible - on PS5/Xbox you might also try to do this in-game: go to Settings, Profile and in bottom left "Delete". I don't know if this will work, I only did it through appdata. There is a risk this will delete your career saves, and tournaments, but I think they should be saved in the cloud, not sure though.
Then when I started the game again I was able to make changes to the squads.
For me the bug started when I tried to make changes to squads in the squads menu that you access from the main menu after start screen. The bug also affected only the teams that I tried to edit in the squads menu.