、。
In short, you can't trust that man.
Literal
Essentially, that man [topic-は] trust-can-not.
信用できない is 信用する ('trust') + できる potential ('can') + ない (negative) — 'cannot trust.' Note the difference from 信用していない ('is not trusting [now]'): できない frames the inability as categorical, while していない is the speaker's current state. A short, blunt verdict on the man's character.