some observations
- first do the setup, then start the service
- also move the notifier down (see arrow) and in case no GPS is enabled, show a notifier which tells the user to enable GPS
- also it might be a good idea to set the request header as @SteveJG showed you in his example here HOW TO: use LocationService extension Version 1 and Version 3 with a GoogleSpreadsheet to record real time locations while the app is in the background - #7 by SteveJG
- it does not make much sense to set the bckground web property first to false and later to true... only set it to true
- use a reasonable value for the distance interval... this property determines the minimum distance interval, in meters, that the sensor will try to use for sending out location updates... 0 does not make sense... try to first use the default value of 5... and after you got it running, then later you can adjust it to your needs...
- it would be wise to use the ErrorOccured event... see also Q2 here App Inventor Extensions: Location Service | Pura Vida Apps
- as general recommendation to test your POST method: as test use the web component to send some test values to your Google spreadsheet to find out the correct blocks for the POST method... after you got this working then setup the background web functionality accordingly...
Taifun