When a creature has the Truesight ability, it does more that see through magical darkness or see invisible creatures. It can also see the true form of shapeshifters (pretty handy that), or a creature transformed by magic (polymorph, disguise self).

A creature with Truesight can see into the ethereal plane while still seeing things on this plane of existence (cool, though not used all the time). Most of all, any visual illusion can be see as an illusion.

However! Phantasmal force does not create a visual illusion – it creates a physic one. The illusion is in the creature’s mind, and thus truesight cannot help it avoid the spell’s effects.