Is it possible to have offline fm radio on Nexus 6p?

It depends on how it’s implemented, your carrier and phone model if it can be re-enabled. Certain variants of FM Radio compatible phones are known to be software disabled, but keep the hardware (primarily AT&T phones). You can’t fix this on AT&T phones since they’re infamously not open to root and custom ROM installation and go out of their way to actively stop it. There’s a slim chance the AT&T Nexus series will be different, but don’t hold your breath. If you have one of these phones, you need an app like iHeartRadio. I ran into this on my LG G4 H810 (AT&T) since it’s one of those FM Radio phones with the feature disabled.

If it’s available and isn’t blocked, these phones typically use headphones or the chassis for signal. Headphones work better for this job, as the chassis isn’t as good for radio use.

The way the offline FM radio works creates this problem by design (that, or the carrier is kind of crappy and does it on purpose to get more data money) :(. It isn’t your fault that the actual hardware radio is present but remains unused despite costing nothing to enable.