Hi, many things can be the cause. Firstly, make sure you haven't exceeded the number of players in your first team. Secondly, a red rectangle with a small stop sign icon indicates that the player is unavailable for the upcoming match. This may be due to:
- league rules (limitations),
- an upcoming match is being played against his home team,
- there are competitions being played in which the player is participating (for example, a call-up to the national team),
- the player was banned from a future match for violating the rules.
In addition, a dark blue color of the player's name indicates that the player has been loaned to another team, while light blue indicates that the player has been loaned from another team to your team.