This question already has an answer here:
Wizard 1 casts wall of force which surrounds Wizard 2. Now both wizards have full cover against each other. For example, now they cannot cast fireball against each other, because "nothing can physically pass through the wall" per its rules.
But what about phantasmal killer? From description:
You tap into the nightmares of a creature you can see within range and create an illusory manifestation of its deepest fears, visible only to that creature
I think wall of force does not help against phantasmal killer because nothing is physically passing through the wall, but I wanted to confirm if that's how RAW works here.