Systemmode isn't well supported on nougat. Actually, SuperSU isn't supported, period. Better off using MagiskNo, I have a custom port firmware from Note 7(Android 7.0.1). As always, after the firmware I make backup without root and when something is wrong with the firmware, I restore backup and install fresh Magisk from above.
Well, this time I decided to install SuperSu, but I just can't install SuperSu Systemmode (Systemmode).