"You're out of your element" is an idiomatic expression in English, that means the same thing as "fish out of water". According to Wordreference, "essere spaesato" or the idiom "fuori del proprio elemento" translations of "you're out of your element". "Fish out of water" can be used as well, but since both expressions have their own idioms in English and Italian, that is probably why Duo did not accept your answer.
You can say both "dell'Italia" and "d'Italia".
When Mameli wrote the anthem (about 1850) "d'Italia" was more used, today dell'Italia is more frequent.
If you put in google "dell'Italia" and "d'Italia" you can see and compare the context.
For instance, when a recent strike blocked Italy's highways, all newspapers unanimously wrote: Il blocco dell'Italia continua (Italy's blockage continues).
Nowadays the problem is often skipped using the adjective whenever possible. For instance, while in the 19th century you could easily read "I docenti d'Italia" (The teachers of Italy), nowadays you read "I docenti italiani". Anyway, the first is correct also.
The same goes with the idiom of this thread.
Sei come un pesce fuori dall'acqua.
You could have written:
Sei come un pesce fuor d'acqua.
Both indeed are correct.
It's not my cup of tea indicates that it is something that others like, but you don't: John loves to get up early for a long bike ride, but that's not my cup of tea. Fish out of water is more that you feel uncomfortable: I felt like a fish out of water when I realized that everyone else in my new school buys their clothes at very expensive stores.
I wrote "You're a fish out of water" which is how I'd say it and was marked wrong because "You are missing the article 'a' here." Huh?? Maybe it would have been better to include 'like' or 'as' in order to make it a proper simile like it is in the Italian, but I've always said it as a metaphor. Regardless, you wouldn't say "You're a fish out of A water." That's just bad English. Water is mass noun, so it's never singular. Anybody else have this problem?
come is an adverb, it can introduce terms for comparison or mean "the same as". es: "forte come un leone"="as strong as a lion" , "sei una brava persona, come me"="you are a good fellow like me".
dalle is preposizione articolata (i.e. preposition+article) "da"+"le" (or the correct article according to the following noun). da is the preposition that usually indicates the origin of something (from, since, etc). es. "vengo da Milano"="I come from Milan" , "ti amo dal primo istante"="I love you from the first moment".
"out of" = "fuori da"
both "come" and "da" can have some other meanings, but I don't know what very confuses you. If you have other question, ask!
PS: idioms are not the best thing for learning grammar: they usually follow their own rules.