"fool" can be used as an adjective to describe a person. I'd never use it that way though. I don't get it. That fool son of ours is sacrificing his career for this new girlfriend. I don't have time for pointless/meaningless/vapid conversation with you.

To make the grammar correct, you need to use the adjectival form "foolish." It would also be more natural here to say "the time" instead of "free time." "I don't have the time for foolish conversation with you." By the way, this is not a friendly or polite thing to say. It sounds as if the speaker is irritated, annoyed, or even angry.

Can I say: a crappy conversation?

As DavidK pointed out, I guess you can in fact use "fool" as an adjective in very restrictive cases, for people or a "thing" as in "damn fool thing". But it also sounds a bit to me like something Foghorn Leghorn would say.

Then maybe "[...] for this pointless conversation with you". But like I said, the adjective for "fool" is "foolish". So you could say "foolish" as well if you want. Just not "fool".