Please take care to never park without an active session.
Even though a parking space may appear visibly free, if the Parkable app is unable to allocate you a space or you are unable to start a session, there are few reasons for this:
All of the spaces you can see are already allocated to another parker who is yet to arrive.
The carpark may be temporarily closed for maintenance, construction or an event.
If your location service's are turned on in your phone, the Parkable app can show you other car parks nearby that may have spaces available.