I bought an Apple Watch series 7 from someone. He remove it from his iCloud account, I tried to pair it and I couldn’t

He didn't remove the activation lock;

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.