stolen iphone 8+

If it is activated, you should be able to track it when it is turned on and connected. If you can't do it, the device is turned off or "find my iphone" is not activated.

Read these support guides:

Find My iPhone, iPad, Mac, and Apple Watch - Official Apple Support

and/or

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Regards

Giulio

You can check that by going to iCloud.com.

Log in with your Apple ID, click 'Find my iPhone'. If your iPhone 8 Plus os on the device list, then it means that this option was enabled.

If it's not, then no and there's no way to enable it.