I lost my watch can’t find it

Erase it remotely but do NOT remove it from your account.

If you have already checked the Find My app on your iphone and it does not show up then there is little else you can do to locate it. I doubt you will see it again. If you had wrist detection and passcode on then your data is safe. 

The following article explains about the "Find My" options and other actions you can take 

If your Apple Watch is lost or stolen – Apple Support (UK)

Read this support article about what to do if your Apple Watch is lost or stolen --> If your Apple Watch is lost or stolen - Apple Support

Manage your devices in Settings on iCloud.com

You can use Settings on iCloud.com to […] mark your Apple Watch as missing.

iCloud User Guide - Apple Support