Mueller concluded that there wasn't enough evidence to prove criminal conspiracy.Mueller concluded that there wasn’t enough evidence to prove collusion.
"Collusion" is not a criminal charge. The collusion that indeed took place was detemined not to warrant a criminal conspiracy charge, in part because of a standing protocol of not charging a sitting president with a crime.
The collusion hoax is the belief that Trump did not collude with Russia, on the basis that he was never convicted of collusion, though that is something you can't be convicted of. It's like declaring that Trump has never farted, since he never has been convicted of farting.
It was a brilliant red herring to throw out there, that he wasn't convicted of something for which you literally cannot be convicted. All the rubes swallowed it, hook, line, and sinker.