Hi ema110,
You started from a great resource: Share your Activity and compete with friends with your Apple Watch
If you've completed the steps in the article be sure to also test the section below found in the same article.
"If you see an error when you try to add a friend or send an invitation
Sign out of iCloud on your iPhone, then sign back in:
- Tap Settings > [your name].
- Scroll down and tap Sign Out.
- Go back to Settings > Sign in to your [device], then enter your Apple ID and password.
- Tap Sign In.
- Try to add your friend or look for the invitation.
If you still can’t add a friend, make sure that they have an Apple Watch and that you haven’t added the maximum number of friends. You can add up to 40 friends."
If the issue continues, reach out to Apple Support directly to further assist.
Contact - Official Apple Support
Contact Apple for support and service - Apple Support
Best.