Register Players and Coaches for the National team
On the list of players or coaches, only those who were previously assigned as national team players/coaches will appear. Therefore, before assigning them to the match, they must be marked as national team players/coaches.
To do so:
Open a player/coach card
Press the Options button → New registration (National team) button.

Inspect the National team registration details and hit the 💾 Save button

Once you have done that, the player/coach is marked as a national team player/coach, and it is possible to assign him to the match.
If a player/coach, for any reason, can no longer be part of a national team, they can be deleted in the same way they were added.