There are a few reasons you may not be able to start a parking session in a zone:
The zone is not currently active (for example, parking is free at that time)
The zone is restricted based on time of day, permit requirements, or local rules
Parking is not allowed at that location during the current time window
If payment is required, the app will prompt you to start a session. Always follow posted signage, as it reflects the rules for that location.