Replaced my 13 pro. Watch will not pair

iPhone 6 - Did you mean Watch series 6?

Put your watch on its charger and keep it there until you complete the erase. (select the watch OS version)

Erase Apple Watch – Apple Support (UK)

Erasing all content and settings this way erases all media, data, and settings from your Apple Watch, but it does not remove Activation Lock if it has one.

Or if it is still under your iCloud account you can erase and remove it from your account in Find My or https://icloud.com/find .

Wait for the process to finish, then set up your Apple Watch again. Set up your Apple Watch – Apple Support (UK)

When asked, I suggest you set up as new.

If it doesn't help you can unpair again and use the backup.

If it fixes it you have a choice to make , you will not lose any data that had been syncd to iPhone activity, but you will lose faces and some settings .