Hi
If you have not yet done so, then it may help to update your iPhone and Apple Watch to the latest compatible software versions, starting with your iPhone:
- Update the iOS on your iPhone, iPad, or iPod touch - Apple Support
- Update your Apple Watch - Apple Support
Also install any available update for your Barclays app via the App Store > Updates on your iPhone.
If the issue with using their app with Apple Watch persists, then I suggest that you consult the developer's support resources and/or contact them for assistance.