"Я не знала, що ти міцний горішок."
Translation:I did not know that you are a hard nut to crack.
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Yes, this is allowed, and it happens automatically on Duolingo: if the translation given as acceptable has "did not" and you input "didn't", Duo accepts it, we don't have to add it as a separate option.
It means you did something else wrong. Please, include the whole sentence you inputted.