I received Apple watch bought 3rd party and I can’t sync

Make sure the Apple Watch is charged and in close proximity to your iPhone. Open the Watch app on your iPhone, tap "Start Pairing," and follow the on-screen instructions. If issues persist, consider resetting the watch and trying the setup process again.

What version watch and what iPhone (on what iOS version)? If your iPhone has iOS 17 but the watch has anything lower that watchOS 10 then that would be the problem. Otherwise are you getting any kind of error message?