Why does bootcamp say this?
The startup disk must be formatted as a single Mac OS Extended (Journaled) volume or already partitioned by Boot Camp Assistant for installing Windows. I don't know what it means or how to fix it. Thank you.
It means that to use BootCamp to install windows on your Mac, you need to have a single partition on the internal drive when you use the Boot Camp Assistant to create the windows boot partition.
to "fix" it, you backup all your data on your computer, then you delete all the partitions except the MacOS boot partition. You then ue the Dsk Utility to enlarge the one remaining partition to consme the whole drive. Following that, you use the Boot Camp Assistant to carve off the section of the hard drive you want to dedcate to Windows, and follow the instructions from there.