I found two issues. The first of which I realized have been present in FMRTE/FM for some time, even back in 2017. I moved a player from Inter Milan to my club, Bayern Munich, but there are now two copies of the player, one in each club. This causes the AI to play my player in matches, causing him to be inelligible for my matches. His value stays at 0 and ending his contract doesn't do anything, nor does moving him to inter, ending his contract etc.
I tried solving this issue by adding an unavailability to him by banning him from playing in the Serie A TIM and/or in the Italian Domestic League.
In the screenshot, you can see that I added an unavailability. However, after looking for Seria A TIM using the search tool and selecting the league, the league does not persevere to the box. The box simply stays empty. No matter how many times I select using the search tool and clicking OK, it never moves the data to the Item box. The item box always stays empty.
I'm on the latest updated FMRTE, so I don't think that's an issue. I'm also running FMRTE as an administrator.