Why can't I sign my youthplayer in the academy for my first team?

In order to take a player from the academy into your team, you must first have scouted him 100%. Use the "SCOUTING" actions on the player to scout him. The yellow scout progress bar underneath the player indicates how far he has already scouted. On the screenshot below, player "CIESLAR" is ready to join the main squad, while the rest still need to be "scouted"