I've seen multiple people report this issue, and I still haven't seen any official response from EA.
The Youth Academy Showcase matches are completely broken – players are locked and cannot be swapped or changed. This is a basic feature of the mode.
I paid full price for this game, and it’s frustrating to see a major bug like this go unnoticed.
Reinstalling or resetting doesn't fix it.
EA, are you aware of this issue? Is a fix coming in the next patch?