Can you track an iPhone when it's on airplane mode?

As long as the person who stole it does not know the Passcode, there is no way they will be able to get into the phone. It will always be locked to the person that has their Apple ID signed in.

With it being in Airplane mode, it will prevent you from tracking the device or remotely erasing it, but does not affect the security measures on the device itself.

With the device in Airplane mode, it will show as Offline until Airplane mode is switched off.

There is nothing they will be able to do with the device as long as they do not know the Passcode. The odds of being able to guess the Passcode before it is locked from too many attempts is basically nil unless the passcode was something like "1111" or "1234".

Even if they attempted to reset the device, it would still be locked with your Apple ID and will not shut off Find My. In fact the reset process would require an internet connection, so you would see the location updated if they attempted to do that.

I suggest you just do what you can such as "Erase" and "Mark as Lost", but do not remove the device from your account. Just like the odds of them being able to ever use the device or access your data is nil, the odds of you getting the device back are about the same.

When you do "Mark as Lost", you are given the option to leave a message on the device if found. You want to be careful with this as if you provide a phone number or email, then you can expect to get fake messages from Apple claiming they have found your device and need your Password. This is a scam and when the hackers get your Password, they are able to remove Find My and use the device.