can this be an adjective?

"payment pattern" is a compound noun, so "payment" qualifies "pattern" without having to be an adjective.
"paymnt pattern" and "pattern of payment" do have the same meaning and usage.
Similar situations include:
"apple juice" and "juice of apple",
"English teacher" and "teacher of English",
However, not all compound nouns behave that way. For example, "water melon" is not exactly "melon of water", although it kind of carries that meaning.