When adding a game/practice through the calendar, the current date is used to determine the current Athletic season so the system knows which teams are active and should be available in the list.  If the current date is in between seasons, no teams will show in the drop-down.

To adjust Athletic Season dates to ensure there are no gaps:
  1. Go to Core > Settings > School Information > Years & Terms
  2. Select the appropriate School Year and School Level
  3. Edit the Begin Date or End Date for Athletics Seasons to ensure there are no gaps between seasons