Unless there is a very specific "attempt to acquire CSAM" law then no they're not fucking guilty of any crime. If you live in a state where marijuana is illegal and you smoke some oregano because you thought it was marijuana you're not guilty of actually possessing marijuana.
A criminal law is composed of a number of individual statutes. When a state is trying to prosecute someone for a crime they need to prove three elements for each statute: the criminal act (actus reus), intent (mens rea), and the concurrence of both of those.
If a cop sells you oregano and you think it's marijuana you might have the intent to buy marijuana but there's no actual criminal act because oregano isn't illegal. If you make a law that only requires intent then congratulations, you've created thought crimes.
If you want to make entirely fake CSAM possession illegal, that's essentially the same as an intent-only law and creates thought crimes. It's a slippery slope.