I bought a Apple Watch from someone on offer up but the Apple Watch needs a activation code an the guy won’t text me his email and password so I can unlock it

I can well imagine who would not give someone their Apple ID and password. I wouldn't do that either ;-)

You can ask those from whom you bought the Watch to perform these steps so that the Watch is released:

If you already gave away your Apple Watch, you can erase and turn off Activation Lock from iCloud.com. Here's how:

1. On a computer, go to Find My iPhone on iCloud.com and sign in with your Apple ID. 
2. Select All Devices, then click your Apple Watch.
3. Click Erase Apple Watch. Select Next until the device is erased.
4. Click the X button  next to your Apple Watch. 

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

Another way to remove the activation lock is through this Apple support site. There are conditions attached to this which are described. https://al-support.apple.com/#/additional-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.