To enable root, i think u need premium ver (i got mine
here). You can see the toggle once you open VMOS settings (on the lower left corner). After that, enable root access to the injector by giving access in superuser (hashtag symbol app). It will usually appear whenever you try to open the injector.
If it still doesn't work, restart VMOS (with root access). This worked for me but idk might work for you too hehe