Learn Italian: Quest'anno i carri colorati non sono usciti. - This year the colorful floats didn't go out.

No. Unlike weather verbs like piovere (to rain) which can take either avere or essere, uscire (to go out) is a verb of motion and strictly requires essere in compound tenses like the passato prossimo. There is no flexibility here.

Why does the past participle end in -i (usciti)?

Because the auxiliary verb is essere, the past participle must agree in gender and number with the subject. The subject here is i carri, which is masculine plural, so the default uscito changes to usciti.

What exactly are i carri in this context?

While un carro generally translates to a cart, wagon, or carriage, in a festive context (like the famous Italian Carnevale), i carri refers specifically to the large, decorated parade floats.

Why is it written as Quest'anno instead of Questo anno?

When the adjective questo (this) is followed by a noun starting with a vowel, like anno, the final vowel of questo is dropped and replaced with an apostrophe. This is called elision and makes the phrase much smoother to pronounce.