Translation:I probably did not turn off the computer.
I think I'd not turned off the computer - is not good English. I put 'I don't think I turned off the computer'
"I'd not turned" isn't good English, but "I think I had not turned off the computer" is okay. Unfortunately Duolingo algorithm not only accepts "I'd" for "I had" regardless of the context, but even sometimes suggests it as an answer. Nothing we can do about it.
As for your answer... sounds fine to me, added.
So if you have "This is the sandwich that I had" entered as an answer, then Duolingo will sometimes suggest "This is the sandwich that I'd"? Yikes.
DUO corrected my "I don't think that I turned off the computer." to "I don't think I had turned off the computer.". If DUO needs "had" in the subordinate clause, then really a past tense is also needed in the main clause: "I didn't think [that] I had turned off the computer."
Is "It seems like I didn't turn off the computer" a good translation here?
"Probably I did not turn off the computer"? This feels like a natural word order to me.