Many people get confused about the difference between God existing and God being real, because the words are used interchangeably so often.
To understand why God cannot exist, you must understand what exist actually means. The word exist comes from two Latin words, ex – which means out of, and sistere – which means to stand, or stand alone, as in self being. If God existed, he would have to be from something, in other words, he would have to be created or he would have to create himself.
That would be impossible since God, being eternal, would have to BE before he was. God does not have a beginning, does not age, and will never end. Everything that exists had to come out of something that was already there.
Therefore the original creator, must be eternal. If nothing was eternal, the creation process could never start.