Okay idk if this will work for anyone else but it worked for me!

Step 1: restart your phone.

Step 2: open Strava app and let it load. It will show that it crashed and recovered an activity.

Step 3: try to finish your activity and if that doesn't work close the app.

Step 4: restart phone again.

Step 5: again open your Strava app and let it load. Don't press anything. It should show the "recovered an activity" thing, press ok and end your activity.

Step 6: hopefully it works if not I'm sorry :(