First, make sure you are trying to park inside the operating area. Remember that are not allowed to park outside the orange boundary (service area) or over orange zones.
Also, you need to have your GPS and data mobile on (wifi needs to be off).
If you still can't end the rental of the bike, contact the provider immediately, so they can manually close it for you.
Write them a request indicating your account phone number, the ID of the booking, the minutes you rode and the time you started/finished the rental.
Human Forest
https://www.humanforest.co.uk/contact
+44 20 81 57 74 91 - UK support only
Dott
https://help.ridedott.com/hc/en-us/requests/new
TIER
+44 80 8164 9486 - UK support only