There are a couple of reasons why you may not be able to book any holidays for the next year currently:
1. If you're trying to book Holiday for Next Year, and you've already imported a balance then it's possible you have disabled Next Year bookings.
In the Admin Panel, under System Setup you will see the following setting called 'Disable next-year bookings'. This can be enabled/disabled Globally, or on a Template level for multi-country systems.
2. If you still cannot book Holiday for Next Year, you should check that the user definitely has a balance for Next Year. This can be done by checking the Entitlement Report for Next Year, or by simply checking their balance on the Front-End of the system.
If that user does not have a balance, you will need to update their Next Year entitlement on their profile.
NOTE: If you're looking for information on how to import a balance for Next Year, visit this guide.