Is the following sentence written in a conditional sentence structure?

is this sentence correct? It starts a small paragraph. My question is, does it violate any English rules? If it does, which ones? If it doesn´t, which rules validate it? It sounds perfectly fine to me, but someone told me it did not follow correct conditional sentence structure. I´m a little confused.

The sentence is:

Payments coming from a pension trust account are taxable.

Someone told me it would have to be, "Payments that are coming from a pension trust account are taxable."

The first sentence seems perfectly fine to me. Just can´t find any English sentence structure rules to validate it. Is it fine to omit the verb to be? If it is, then when is it ok to do so? Any input would help.

Number one, the sentence is fine. Passive verbs and their pronouns are often implied rather than explicitly stated: "A Boy [Who Is] Named Sue."

Number two, that's not a conditional sentence. Conditional sentence take the form "If x, [then] y." "If a payment comes from a pension trust account, it is taxable," would be a conditional sentence. Your sentence is a simple logical statement, along the lines of "All x that are y are also z."

This sentence seems perfectly fine to me. You did not omit the verb to be, you said "Pensions ... are taxable".