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 it exists, let alone accurately describe its 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 phenomena within galaxies that have not yet been discovered. But this does not mean that they do not exist since we cannot describe knowledgeably, sufficiently, or accurately.
We know that things exist which are difficult to describe adequately. A rainbow is another good example. A verbal description of a rainbow cannot compare to seeing one. By default, the description is inadequate as compared to the reality of eyewitness encounter. But, that does not mean that rainbows do not exist because we cannot adequately describe them with words.
God can be described though not perfectly. He can be described sufficiently 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 that 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 describe Him perfectly, we can describe Him by listing His attributes and qualities. They reflect the nature of the thing described. Therefore, we can describe God, and our insufficiency to describe Him adequately in certain areas does not diminish His existence nor mean that He does not exist. It simply means that He is great.