A Player cannot be in two simultaneous periods at the same time.

An error message will occur like this:

This will show the concurrent period in which the athletes are already 'Pipped' into. Either the athletes need removing from the current period, or the new period's start and end time must not overlap with the current period in which the players are in. 

If there is no other period available, an athlete might also be 'Pipped' into a period from another Activity that happens at the same time. If this is the case, open the simultaneous activity and delete then player from the periods.

Please see this guide for more information on Periods: How to Create, Edit and Delete Periods