Learn Italian: Il fornitore sa che ciascun prodotto difettoso è un problema. - The supplier knows that each defective product is a problem.

Ciascuno follows the exact same rules as the indefinite article uno. Because prodotto is a masculine noun starting with a single consonant, you use the shortened form ciascun (just like you would say un prodotto, not uno prodotto).

Could I use ogni instead of ciascun here?

Yes! Both ogni and ciascun translate to "each" or "every" and are always followed by a singular noun. Ciascun emphasizes the individual items a bit more ("each single one"), while ogni is closer to a collective "every," but they are largely interchangeable in this context.

Sapere is followed by che, so why isn't the next verb in the subjunctive mood?

We use the indicative mood (è) because sapere (to know) expresses certainty or a concrete fact. You typically only need the subjunctive after che for verbs expressing doubt, opinion, emotion, or hope (like credere or sperare).

Why is it un problema and not una problema if the word ends in -a?