My state is experiencing Doomsday, my VRF is at level 8 but I can’t conquer a level 8 tile, why is..

When you try to occupy the plot is should tell you what the level of infection is, if its in the red then you will not have enough immunity to safely occupy it. If you r Alliance has more than 1 alliance center then place a second VR building.

Additionally, depending on the strength of your APC you may also not be strong enough to defeat the guardians that protect the plots. Even if your immunity is high enough you may not have enough troops. You may be able to complete the occupy with more than one attack though in this case.