Can I send Apple my receipt to get the watch unlocked

Apple will not act on your receipt. They will only do it for the original owner, not a second hand Watch. You need to contact the original owner and have them remove the device from their account to remove Activation Lock so you can use the device. Otherwise, return it for a refund. Right now, the Watch is useless to you.

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.