In playmetrics, teams cannot be created within an academy until the academy team has been set up as a "pool team".

Edit the settings for the team to do this, and to add pools to the team.  Once pools are created, you will find a tab for pool assignments.  Create a pool assignment for the season, and then coaches will be able to set up their rosters.

https://youtu.be/WP0ZQ87JEF8