In a new support document, Apple notes that if you have an iMac (27-inch, late 2012) with a 3TB hard drive, you’ll have to remove its Boot Camp partition before you can install macOS Mojave.
Boot Camp allows you to install Windows on a Mac. Howeve, after you upgrade to macOS Mojave, you won’t be able to use Boot Camp to install Windows on this Mac.r
The alert “Installation can’t proceed with Boot Camp configured” appears only when you try to install macOS Mojave 10.14 on these iMac models with an existing Boot Camp partition. To install macOS Mojave on this iMac, first back up your Windows data, then use Boot Camp Assistant to remove the Boot Camp partition. After the Boot Camp partition is gone, you can install macOS Mojave.