There is, seemingly, a bug in Android Lightroom Mobile. In Android 7 there is a setting called Display size, under the display panel of settings, which effectively sets screen scaling. If this is set to "Small" on my Nexus 6P the camera function disappears from Lightroom Mobile. It comes back as soon as I set Display Size to the default.
Android N is still not official for all devices and we have not tested it extensively on our Apps.
I personally do not have a Nougat compatible device handy with me so will update once I have tested the same.