申し訳ない
もうしわけない · moushiwakenai · JLPT N2 · expression
I am sorry
申し訳ない = 申し訳 (excuse, reason to state) + ない (none): I have no excuse to offer
申し訳 is a humble term for giving an explanation or excuse, and ない means there is none. Saying it admits fault and apologises, so it signals a more serious sorry than すみません.