I bought a watch from a previous owner. They erased it and unpaired it. But the watch is still locked and I can’t pair it with my phone.

Have the seller follow this procedure:

… What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

They can unlock the watch remotely, as described in the section If you already gave away your Apple Watch in that article.

The person that you bought/got this watch from needs to follow all the steps on this page;

Remove the Activation Lock so that the buyer can use it.

If they can't or won't then get your money back or recycle the watch and mark it down as a bad experience.

Neither the users on this forum or Apple can help you,only the original owner can help you.