by -> but
Misguided downvote, imho. (shrug)
"not a document but (rather) a process"
In the former (as OP typed it), it's grammatically suspect but also seems to imply a missing "created" like I inserted. In that case it'd be ambiguous whether the OP feels requirements are not documents, or perhaps they are documents, just not ones "by a process".
In the latter, which I took to be the intended meaning, OP is saying "requirements are a process, not a document."
The "not X but Y" is grammatical and clear, equivalent to (boolean pseudocode) "Y && !X".