大変申し訳ございませんでした。
I am deeply sorry.
✦ AI pragmatic read · suggests Kenjougo
A highly deferential apology used in formal or business settings, typically spoken by a subordinate or service provider to a superior, customer, or someone they wish to show great respect toward.
A starting point, not the final word — open the entry to add your own note if you'd explain it differently.