There is a limit to the number of Trained Players you can hold, and you cannot register more than this limit.
To train a new Player, you will need to "Transfer" a Trained Player to free up space.
■Steps
[Team] > [Trained Players/Formation] > [Trained Player List] > [Transfer]
Please note that the data for a transferred Trained Player will be deleted.
If the issue is not resolved, please contact us here.