Your phrase could refer to a few different things depending on the context:
It could refer to Kevin Hart’s memoir, I Can't Make This Up: Life Lessons , where he shares real-life stories that are almost too ridiculous to be believed. Cant make this Up
It might be a comment on Lee Gutkind’s book, You Can't Make This Stuff Up , which is a guide to writing creative nonfiction —using literary techniques to tell true stories. Your phrase could refer to a few different