That's an interesting way to phrase it. I get that this is a play off of the "This X doesn't exist.", where X is a machine-generated entity such as a picture of

That's an interesting way to phrase it. I get that this is a play off of the "This X doesn't exist.", where X is a machine-generated entity such as a picture of a cat or the bio for a person. If by "chemical" you mean "substance (as represented by this molecule)" and if by "exist" you mean "hasn't been made yet," then it might make sense. "This substance (as represented by this molecule) has not been made yet" doesn't have the same ring to it, though. Molecules are abstractions. Leaky ones at that.