Can you write an epilogue without writing a prologue?
Yes and no.
If we take the words epilogue and prologue in a very literal way, then they mean something like "after word" and "before word". So if you write something that consists of two parts then the first part could be called a prologue to the second and the second an epilogue to the first. In that sense, it is not possible to write an epilogue without making what went before into a prologue.
Treated a little more sensitively, an epilogue is an addendum to the main piece of writing which provides additional explanation or conclusion, while a prologue is an introduction. So we would not normally apply the term "prologue" to a main piece of work that happens to have a - typically somewhat shorter - epilogue.