Cannot Remove Bootcamp Partition

I tried removing the bootcamp partition but it gave me this error so please advise as soon as possible. "The startup disk cannot be partitioned or restored to a single partition , the startup disk must be formatted as a single Mac OS Extended (Journaled) volume or already partitioned by Boot Camp Assistant for installing Windows."

MacOS Mojave 10.14.3

MacBook Pro (Retina, 15-inch, Mid 2014))