Find My: Device or account is not compatible with this feature

That message showed up for me when I tried to use the Find My app to find my mom’s phone.

Her “Find my iPhone” was actually disabled. I turned it back on, and the problem went away.

On iOS 18 the setting can be found in Settings -> iCloud-> Find My -> Find my iPhone

I would also check that “Allow Location Access” is set to “while using the app” and “Precise Location” is enabled under Settings -> Privacy & Security -> Location Services -> Find My

I'm going to assume you ALL have your own unique Apple ID? If yes, you need to send him a request to permit you to see his location, which he has to accept. Have you done that in the People section of Find My? Once accepted, he has to have Location Services turned on his iPhone. If he doesn't want to be tracked, the simplest way is to turn off location services on their device (and I'm not suggesting he doesn't want to be tracked, btw).

See this for more information --> Locate a friend in Find My on iPhone - Apple Support

It could, but without knowing what he has configured on his phone, it would be impossible to diagnose. My guess is as long as he's connected to the same network you are, you can see him. Once he leaves the house and isn't connected to the same network, you can't. And that may mean he has location services turned off on his phone. You'll need him to check on his phone.

Settings > Privacy & Security > Location Services