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Another answer is "I don't have the strength", which Duolingo accepted. For example, "I don't have the strength to go on".
I wonder if "I'm tired" should be acceptable. Poles often say "Nie mam sily" where English speakers would just say they're tired (physically). Like at the end of a hard workout.
It could have pretty much the same meaning (but the Polish sentence more likely means that I just don't have strength right now), but it's a different sentence: Jestem [słaby/słaba].