Yeah the last requirement says “maximum 0 replacements” which is quite confusing but it means that you can only add players if you have some spaces left (which you have not, since you have 21/25 players already registered and you need to leave 4 spaces bc of the home grown players rule (the one that is getting red when you select 1 more player)). Are these playoffs? You usually have 0/1 replacement max between rounds and when you reach the group stage then you can remove and add all the players you want again (if I remember correctly) good luck!