If I purchased a Apple Watch second hand and the person doesn’t remover the activation code what do I do?

You need to get in touch with the previous owner of the Watch, otherwise, the Watch is of no use to you. You should ask them for a refund.

About Activation Lock on your Apple Watch – Apple Support

The person that you bought this watch from needs to follow all the steps in this link;

Prepare the watch so the buyer can use it.

If they can't or won't then get your money back or recycle the watch and put it down to experience.