How can I get my watch to connect if it’s saying it’s connected to someone else iPhone

The other persoon needs to unpair it. Unpair and erase your Apple Watch - Apple Support

When the Watch is paired and activation locked, See this link and follow the instructions How to remove Activation Lock - Apple Support

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.