After you flash and reboot, install Magisk Manager 5.6.3 or 5.6.4. Then you need to get into Magisk Manager and install Magisk 16.0 (shown near the top of the menu). Do not upgrade to Magisk Manager 5.7 unless you have TWRP installed and can do this through the custom recovery.
I'm running Magisk 16.0 and Magisk Manager 5.6.3 with full functionality, SafetyNet, and Root.
Flashed, rebooted and installed version 5.6.4. when I enter magisk manager i get this (see screenshot)
Don't have install option. What could have gone wrong?!