The two structures shown (left and right) are stereoisomers of 2,3,4-trichloropentane. Both are achiral. They are related as

In both structures C2 and C4 have opposite descriptors ($R$ and $S$), so each has a mirror plane through C3 and is a meso compound. The two differ in the orientation of the Cl on the pseudoasymmetric C3 relative to the other two chlorines: one is (2R,3s,4S) and the other is (2R,3r,4S). An achiral compound is its own mirror image, so two different achiral compounds can never be enantiomers. They are stereoisomers that are not mirror images, i.e. diastereomers.