It is clearly stated that i do not have to register the u21 player for the European Champions League. I have register 25 players including the 8 homegrown players. I have few of u21 players in my team and decide to not register them as the rules said that the u21 players can play for UCL match without being registered. But, all of this u21 players become ineligible.
when your registering your team they should have a "u21" sign besides them.
there is usually a date like "under 21 before the date of 23203.23092-230320" or something. so they might have exceeded that or something?