Learn Polish: Dziękuję za twoją szczerość.

It is an abstract noun created by taking the adjective szczery (honest, sincere) and adding the suffix -ość. All Polish nouns ending in -ość are feminine.

I know dziękować za requires the accusative case, so why doesn't szczerość change its ending here?

You are right that za takes the accusative here. However, feminine nouns that end in a consonant (like all words ending in -ość) have the same form in the nominative and accusative cases.

If szczerość doesn't change its ending, how can we tell it is in the accusative case in this sentence?

We can see it in the possessive pronoun twoją. Because szczerość is feminine and in the accusative case, the pronoun must match it, taking the feminine accusative ending -ą instead of the nominative twoja.

Could I use prawda instead of szczerość in this context?

They mean slightly different things. Dziękuję za twoją szczerość thanks someone for their character trait or the act of being honest (sincerity). If you say dziękuję za prawdę, you are thanking them for the specific piece of factual information (the truth) they provided.