flash stock rom then boot to homescreen before you proceed to flashing and boot philz [after flashing stock, you don't need to flash magisk uninstaller anymore]
so from @hovatek's reply
1. Boot to Philz recovery
2. Skip
3. Skip
4. Flash Magisk beta, last one version 16.7.zip
5. Wipe Davlik cache
6. Reboot
7. Launch Magisk Manager and open Modules
8. Download (don't install) the 'f2fs work-around' and whatever modules you want
9. Move the modules from magisk manager folder to Download folder
10. Boot back into recovery
11. Flash the 'f2fs work-around' immediately followed by your modules
12. Reboot
13. Open Magisk Manager and check your Modules.
lets know the outcome and if there's any errors, state the exact point you get the error. for screenshot, you can use another device to take pictures of your phone screen