With a pass that auto-renews, we don’t allow customers to book further ahead than what they’ve paid for. It is most likely, therefore, that they are trying to book a session past the renewal date of their pass. They or you as an admin can always check the exact date and time of their scheduled renewal in their purchased package detail (Profile>Package Purchases for customers, CUSTOMERS>click on customers name>PURCHASE HISTORY>Packages>click on package name for business admins).
Should this create a logistical issue for a customer you can always expire their instance of the package from PACKAGES>click on package from appropriate filter>PACKAGE CUSTOMERS>click on name>EXPIRE THIS PACKAGE.
If the validity period of the pass does not explain the issue please check that the related session and package have the same Tag and that "Passes" are set as a valid payment method for the session.