"I have no strength" should be correct
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I would really like more notes: I realise this is beta and I'm enjoying it, but a clear briefing about the various case endings when they come up would be helpful.
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.
The meaning may be close, but it's still a different sentence.
Would I am weak work
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].
Ale Jan Błachowicz ma
I know this isn't an English course but we would need a qualifier in this sentence to make it sound right eg "I do not have the strength" or "I don't have any strength" or "I don't have enough strength"