This is one of those forms where the singular is "la chiave" - feminine and the plural is "le chiavi" - still feminine. I have found a number of abstract nouns like this. For example, competition is "la competizione," a feminine singular ending in "e"; the plural is "le competizioni." Until you catch it, this is very frustrating. I think this is why Duolingo tries to put the singular and plural of such nouns in the same lesson. Keep an eye open, and you'll find more as you go along. Buona fortuna.