I bought an Apple Watch second hand and I can’t log in with my apple I d

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

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.

You can't. Only the owner of the account it's locked to can remove the lock.