Whether or not something exists does not depend on whether or not it can be described. If something exists, it exists independently of someone's ability to describe it. That is the nature of existence; that is, things exist whether or not someone knows they exists--let alone accurately describing their existence. Furthermore, something that exists is not limited in existence by the inability of someone to accurately describe its existence. For example, there are galaxies and phenomenon within galaxies that have not yet been discovered. But just because we cannot describe them knowledgeably, sufficiently, or accurately, it does not mean they do not exist.
We know things exist which are difficult to adequately describe. A rainbow is another good example. A verbal description of a rainbow cannot compare to actually seeing one. By default, the description is inadequate as compared to the reality of eyewitness encounter, but it does not mean rainbows do not exist just because we cannot adequately describe them with words.
As far as God is concerned, He can be described--though not perfectly. He can be sufficiently described, so that we can gain an understanding of His nature, greatness, and differences from ourselves. Though we will always fall short when trying to describe an infinite being, we can still say enough about Him to convey who and what He is so that the concept of God can be communicated. We can say He is infinite, gracious, loving, all-knowing, all-powerful, all-present, holy, pure, righteous, that he is not flesh and bones, the only uncaused and infinitely eternal being in existence, etc. Though we may not be able to perfectly describe Him, we can describe Him by listing His attributes and qualities. Attributes and qualities reflect the nature of the thing described. Therefore, we can describe God; and our insufficiency to adequately describe Him in certain areas does not diminish His existence nor mean that He does not exist. It simply means that He is great.