Is it possible to say "Maximally great being doesnt exist"?
No, it is not possible to say.
Consider the basic problem in the statement. the statement claims to contain a maximally great being and also a claim of its non-existence. Obvious problem is that in order to contain a maximally great being, even as an idea, would require ability to contain maximally great idea.
But logical problem is this:
A = "Maximally great idea" and B
So A as an idea becomes bigger than a maximally great idea.
In other words, it is not possible to say that a maximally great idea doesnt exist, because to do so would require statement to contain both maximal greatness and more than maximal greatness, which is not logically possible.
In other words, it is not possible to claim that maximally great being doesnt exist. It is not even possible to lack belief in maximally great being, or establish such.