I can’t remember my old Apple ID and password that was paired with my watch.

Check your web browser autofill entries and other records, or use Apple's iForgot service, or contact their Account Security team, or if you're the unit's original owner and have encountered the activation lock, click here and submit an unlock request.

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You can ask apple to remove the activation lock online on this page;

Remove the Activation Lock Here.

Or this;

If you can't remember/gain access to the apple id that paired the watch to the phone.Take the watch and it's receipt with the serial number along with some photo id and any other proof of ownership to an Apple Store and request that they remove the Activation Lock for you.