Some locations have limits to how much parking you can purchase at one time. Most of the time, they are associated with a city by-law, and HONKs system only enforces this bylaw (or rule) by not allowing you to purchase until 30 minutes after the expiry of the max reached session.
Please move your vehicle, or go out to the machine to purchase more time.
If this is not the case and you are in a private lot or know you should be able to extend, please see Extension Questions