Apple Watch not sync activity with phone
On your iPhone, in the Watch app, go to: My Watch > Privacy > Motion & Fitness - turn Fitness Tracking off. Restart both devices by turning both off together, then restarting your iPhone first. Return to theFitness Tracking setting and re-enable it.
I have just done the above and it didn’t work however I turned aeroplane mode on on my watch for 2-3 minutes and it’s worked. Great thanks Apple
Apple Watch Series 5, watchOS 7