A formal, self-deprecating apology meaning 'I have no excuse' or 'I am terribly sorry'. Used when one feels deep regret and acknowledges fault.
I am terribly sorry for causing you trouble.
I feel inexcusable for this outcome.
General apology; 申し訳が立たない is much more formal and expresses deeper regret.
Standard kanji spelling for this fixed apology expression.
Kana spelling of 立たない; common in everyday writing when the kanji is not needed.
From 申し訳 (excuse, apology) + が + 立たない (does not stand), literally 'an excuse does not stand', meaning one cannot justify one's actions.