boot camp trouble

i have a mac book pro retina early 2015 2.7ghz intel i5 8 gb 1867 mhm ddr3 mojave version 10.14

i had to install windows 10 on my mac using boot camp assistant but due to some error it quit later on it said clearing the partition and nothing happened and the application quit after that when i tried to reopen boot camp assistant and clicked on continue it said 'The startup disk must be formatted as a single Mac OS Extended (Journaled) volume or already partitioned by Boot Camp Assistant for installing Windows.' now idk how to erase this