。
I can't understand this guy.
Literal
I [to-は] this-guy [subject-が] not-can-understand.
こいつ ('this guy') is the proximal counterpart to あいつ — a casual/rough third-person pronoun for a person nearby, carrying dismissive or familiar overtones. 理解できない is the negative potential of 理解する ('to understand'). More formal than 分からない, 理解する implies conceptual comprehension rather than just information.