Player selection shall be conducted using the ' Team Selection' tab of the target league. It will only be possible to change the team selection either for the ongoing matchday or for upcoming matchday, as applicable.
To add a new player to the team, simply press the green 'Add Player' button on top of the player's table, which will display all players you own. In that screen, simply press the green '' button, when available [FAQ 6.14]. Once the player is added to your team, you may also manage him, for instance adding more shares or selecting him as captain [FAQ 6.16].
There may be many reasons why you cannot add a player, being the most common one the fact that his team is already playing or has already played for the applicable matchday [FAQ 8.16].
Another common scenario is that, considering the league positional constraints, you do not have free spots left to add a player with the target player's position [FAQ 6.11]. In this case, depending on the league setup and timing, you may try to remove a player from the same position first and, then, add the target player to your selection.
It can also be the case that the player is eligible for selection but all of his shares you own are already subject to another use (e.g. used in other league or listed for sale [FAQ 4.17]).
To support you in understanding these limitations, you may refer to the icon/data that is displayed in the right end of the row (i.e. in the location of the green add button), as well as to its tooltip which details the constraint you are facing.