"conclusional" exists as an entry in some dictionaries, but I've never seen it in use, and I'm reasonably widely-read. Another adjective derived from "conclude" is the participial adjective "concluding". It is much more common. "He gave a few concluding remarks."

Bit those things aren't really linked. A conclusion is not the same as something that is conclusive..

Hi, I would say, the correct one is conclusive, I never heard conclusional. You can find "conclusion" as well as a noun, but "conclude" is a verb (it means to "end" something)