The pairing between my apple watch and my iPhone is working for everything except the activity. Don't know why, the problem started when I had to re-inizialize my iPhone after a bad block.

Hi

The following steps may help:

On your iPhone, go to: Settings > Privacy > Motion & Fitness - check that both Fitness Tracking and Health are enabled.

On your iPhone, in the Watch app, go to: My Watch > Privacy > Motion & Fitness - check that Heart Rate and Fitness Tracking are both enabled.

Restart both your iPhone and your watch, turning both off together, then restarting your iPhone first:

• Restart your iPhone, iPad, or iPod touch - Apple Support
• Restart your Apple Watch - Apple Support

Unpair and re-pair your watch:

• The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app. Choose to Restore from Backup during set up. Most data and settings will be restored, with some exceptions (eg Apple Pay cards, passcode).
• Unpair your Apple Watch and iPhone - Apple Support
• Set up your Apple Watch - Apple Support

Check also that Wrist Detection is still turned on:

On your iPhone, in the Watch app, go to: My Watch (tab) > General > Wrist Detection.