Learn Italian: Nessuno ha decifrato il segreto della pergamena. - Nobody deciphered the secret of the parchment.

Because the negative word nessuno (nobody) comes before the verb. In Italian, when you start a sentence with a negative indefinite like nessuno, niente (nothing), or mai (never), you do not use non. The word nessuno does all the negative work on its own.

Can I put nessuno after the verb instead?

Yes, but you have to change the grammar. If a negative word comes after the verb, you must put non before the verb. It would look like this: Non ha decifrato nessuno il segreto... (Literally: Not has deciphered nobody the secret...). Both mean the same thing, but starting with nessuno is very common and often sounds more natural.

Why use della instead of just di for "of the parchment"?

Della is a combination of the preposition di (of) and the definite article la (the). Because pergamena is a feminine singular noun, it takes the article la. When di and la bump into each other, they merge into the combined word della.

The English translation says "deciphered", so why does the Italian use ha decifrato (has deciphered)?