Hello @samerrington123, 
Thanks for your post. Please go ahead and delete the Strava application from your phone and power your device off for 10 seconds. Once this is done, turn your phone back on and reinstall the Strava application. You will not lose any data if you do this since all data is stored on the Strava servers.
 
Google Play Store:
Strava: Track Running, Cycling & Swimming
How to update your apps on Android: Support - Google
 
Apple App Store:
Strava: Run, Ride, Swim
How to update your apps on Apple: Support - Apple
 
Hopefully this will solve the issue you are having. If you are still experiencing this issue please submit a support ticket and include the file of the videos you are trying to upload as well as a link to the activity you are trying to add the video to.