I am trying t locat my other iPhone I can see it on my list when I select it show (find my iPhone OFF) I cannotturned on do not know why?

If Find My is disabled on the iPhone, or if the iPhone lacks coverage, or if the iPhone battery is exhausted, or if the iPhone is powered down, it will not provide a location.

There is no means to activate Find My remotely, if it is not already enabled.

There is also no means for users to track by serial number or IMEI.

If the iPhone has been lost, see the following link > If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support