I cannot say (I cannot say whether he will come. – Я не знаю, придёт ли он.); I couldn't say (whether he will come; придёт ли он); for aught I know (понятия не имею); don't know (gennier); I can't tell (Franka_LV); I don't know; actually, I don't know (actually – expletive (filler word) MichaelBurov); I actually don't know (actually – expletive (filler word) MichaelBurov); I don't actually know (actually – expletive (filler word) MichaelBurov); I don't know, actually (actually – expletive (filler word) MichaelBurov)

I wot not (of MichaelBurov)

ah dinnae ken (источник: YouTube video "Gerard Butler Teaches You Scottish Slang SergeyLetyagin)

I have no clue (Johnny Bravo)

I wouldn't know ("[Our cook] Luigi was a stone cold killer back in the day." "I wouldn't know.")

I didn't know! (learningenglish)

я не знаю