。
The two of them aren't getting along.
Literal
Two-people [possessive-の] between [subject-が] smoothly not-going.
しっくり行かない ('not going smoothly, not clicking') uses the mimetic しっくり ('fitting well, clicking') in the negative. 二人の間 ('between the two people') sets the relational scope. The sentence describes a relationship that isn't working — vague enough to cover romantic, professional, or friendship contexts.