Is there a psychological condition associated with an inability to tell the truth?

Some: Antisocial personality disorder is one such condition. It is not really an "inability" to tell the truth, but is an active choice. The person exploits others to his/her own advantage, and if lying gives advantage, that's what s/he does. Also many substance-abusing people do this, especially regarding how much & how often they are using. They also frequently lie to obtain their substance.

Lying: People lie for many reasons, so I would say there is no one psychological condition to explain it. People lie because they're insecure, or antisocial, or trying to save face and avoid ridicule, or protect the feelings of others.......and so on. No one single reason.