There is a problem here. "I can't trust him" is incorrecte and "1 can't trust him" is proposed for the solution.
Woah, that is very strange indeed. Well, the sentence translates to "He cannot be trusted" or "One cannot trust him". It is a defective verb, it's not exactly connected to any grammatical person. It's not "I cannot trust him".
As Duolingo algorithm accepts numbers instead of words (23 instead of twenty three) when translating to English, apparently it showed you "1" instead of "one" because of this and because it was actually the closest correct answer to "I". That's still strange though ;)
It does the same thing with "un" (singular indefinite article and 1, depending on context) in the French course. Apparently it's been reported time after time after time and still isn't fixed...