Doomsday I want to occupy lvl 4 but it says low immunity what should I do to increase immunity?

Immunity comes from the level of your Virus Research Buildings, the honor structure that you also use to heal troops. Upgrade this building and if possible build more of them to increase immunity. (this will need to be unlocked though with speciality points in the blue Construction section_

You can also increase immunity further with speciality points if you have them in the red Combat speciality section. But adding the buildings is the most important part.