If there was nothing before Adam and Eve when did the dinosaurs exist?
According to scientific evidence, based on fossils radioisotopic and other kinds of data, we know that dinosaurs lived from about 240 million years ago until 65 million years ago, at which time it appears that the last of the dinosaurs disappeared.
You seem to be assuming that there was nothing before Adam and Eve, but I certainly do not agree with this. Perhaps you are taking the "days" of Genesis to be literally twenty-four hour periods of time. Of course, this is a possible interpretation, but it certainly is not the only possible interpretation. In fact, many Christian theologians, such as Irenaeus and Augustine, concluded long before the scientific revolution in Europe that the "days" of Genesis were not literal but were metaphorical in that they represent periods of time over which God worked. If that is true, then it should not surprise us that God does not mention specifically creatures such as trilobytes or dinosaurs or any of the myriad of species which lived millions of years before the first humans. Genesis chapter one is not written as a comprehensive scientific document. In fact, it is a theological statement intended to explain God to his people. To summarize Genesis chapter one extremely briefly: God created the universe. God created the earth and the heavenly objects, the sea, and dry land. God then created life in the sea, the land and the air. Last of all God created people. This description leaves out a massive amount of detail for obvious reasons, but it is generally concordant with what we know from science. Creatures such as bacteria, trilobytes, dinosaurs and so forth which the Jews had no experience whatsoever of are not mentioned, both because they did not have vocabulary for these things and because it would have been confusing to unnecessarily mention creatures which they did not even know about.
So, the Bible does not help us at all with the question of when the dinosaurs lived, for obvious reasons, but scientific data tells us that they lived 240-65 million years ago