I sold my ex’s Apple Watch and the new person that purchased it can’t log in. It says it’s still connected to the old user. Can I please get help on how to delete the old user?

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

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. 

Learn more about Activation Lock. 

With a web browser, go to: https://www.iCloud.com and sign in with your Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.

If you do not know your ex's Apple ID & password, that will be a problem. You may have to give the buyer a refund. If the previous owner cannot or will not remove the lock, then the watch is useless to you or anyone else.

Activation Lock is designed to prevent anyone else from using an Apple Watch if it's ever lost or stolen.

About Activation Lock on your Apple Watch - Apple Support

Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner can remove the lock by using their Apple ID & password.

Your ex needs to follow all the steps on this page;

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