Why am I unable to create an assignment past a certain end date?

The system will not allow teachers to create assignments that end past the end date of the subscription for the product in the account.  

So if your product's subscription ends on May 31, 2015.  Teacher's trying to create assignments with an end date past May 31, 2015 would not be allowed to do so.  The assignment would have to end on or before May 31, 2015.

Once your subscription is renewed, you will be able to create assignments up through your new subscription end date.