@Maschio-Alfa
Hi there,
As long as a player is in your starting line-up you should be able to select him for a specialist role, even when you are training that player. If he's not in your starting line-up, then you can't select that player for specialist roles as he isn't going to play in the upcoming match. Also keep in mind that a goalkeeper can only be selected for the captain role and not for the other roles.
If they are placed in your line-up and you still can't select them for any of the specialist roles, then we will need more info:
on which clubslot do you have this issue?
on what platform do you have this issue (Webversion, iOS app or Android app)?
do you get an error message or is that player not listed when you select a specialist role?
post some screenshots so that we get a better view about the issue