It looks like Manchester United wants to develop players from the academy to build a great squad for the future. I haven't heard about such restrictions from other coaches but I think this means the club board is not targeting Manchester United to win the EPL or UC next season. They want Ten Hag to be able to develop players from the academy and shape them into great players in the future. To be honest, Ten Hag is quite good at making strategies, that's why he managed to give the club the FA Cup trophy.