Pairing watch

See Set up your Apple Watch https://support.apple.com/en-us/HT204505. This link should cover your problem unless the watch somehow slipped into activation lock.

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So, just in case, see Turn off Activation Lock https://support.apple.com/en-us/HT201441 and

About Activation Lock on your Apple Watch https://support.apple.com/en-us/HT205009

If you can't turn the activation lock off and want to make it functional, you'll need to contact Apple support and have your purchase receipt for the watch new with its serial number on it. See these support options:

Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

Apple ID Support https://support.apple.com/apple-id

Apple ID account security for country-specific and region-specific help https://support.apple.com/en-us/HT204169

Get Support: We’re here to help  https://getsupport.apple.com/. Go through the options to find the best support option.

See if anything here helps.

Watch -  Set up your.    

Watch - Unpair and erase your.   

Watch - pair with a new iPhone