Apple Watch Series 6 will not sync activity or data

The checks in more detail

so the first questions are whether the two are connected and whether the data you expect is set to sync. Check the connection. Swipe up on the watch to open Control Center, and look for the green phone icon, which shows that it is connected; a red cross means it is not. Make sure that Bluetooth and Wi-Fi are on in the iPhone, that Airplane mode is off on both and that the iPhone is nearby. Restart both devices: on the watch hold the side button and choose Power Off, then restart;

on the iPhone, restart as usual. Open the Watch app on the iPhone and check that the watch shows as connected under My Watch. Update both: iOS on the phone and watchOS on the watch, since the Series 6 is an older model and the newest watchOS may need updated pairing. Then check what is syncing. In Settings on the iPhone, Apple ID, iCloud, make sure Health is on, and that iCloud has free space, because a full iCloud stops Health data syncing.

In the Watch app, General, Background App Refresh and the app's own settings, check that the app is allowed to sync. If a particular data type fails, such as activity rings, open the Health app and look at Sharing and Data Sources. If it still fails, reset the sync data: in the Watch app, General, Reset, Reset Sync Data, which re-syncs contacts and calendars from the iPhone. As a last measure, unpair the watch, which makes a backup, and pair it again; a worn battery may also cause sync failures through low-power behavior. Contact Apple Support if the watch will not pair.