App runs in Simulator but not on watch

Hi! I´m a hobby developer from Austria and have recently created an App that assists Helth Care Professionals when performing CPR.

The source code can be found here: github.com/.../CPR-Timer

 

I tried to side-load this app onto my Garmin vivoactive 3 music but the app won´t start even though it runs perfectly in the simulator. It doens´t display any errors on the watch, it just doesn´t start or do anything at all. The watch doens´t freeze either.

I was hoping to find some information on how this issue could be fixed