if it's a fitbit, those can apparently cause problems. take your watch off and put it in another room, and try and be where there isn't too much bluetooth traffic.
remove all connected devices on the bluetooth section of your phone, turn off bluetooth, restart your phone and keep bluetooth off for the moment. Open pokemon go app. go settings and wait to press the sync with switch thingi. Open up the settings menu in let's go and go to the sync section. Turn on the bluetooth on your phone and press the sync buttons on both devices.
Normally all this isn't needed, but since we're trying to troubleshoot, might as well do it all in one go