This happens because the Gate is getting a signal that the lock mechanism is opened. The first solution is to check if the Gate is locked properly.
Important: A Gate with a 4m arm can have a different lock mechanism.
The second solution is to check the lock connectors:
The easiest way to test a connector is to disconnect it with blue and brown cables, and make a shortcut for that connector. If the Stop error stops flashing, that means that the cable was disconnected from the sensor near the lock mechanism. This step should only be done with allowance of Parklio support.