You might see this error message when the Windows image is too large to fit on the temporary partition or flash drive that Boot Camp Assistant uses to install Windows. For example, Windows 10 Flash install media that's been converted to an ISO file will be too big, because it contains both 32-bit and 64-bit install images.
This error also might appear if you're attempting a new installation using the October 2018 edition of Windows 10 on a Mac that's running an earlier version of macOS.
To fix this issue, download the Windows 10 April 2018 Update from the Microsoft download page. Then use Boot Camp Assistant to install Windows, making sure to use the April 2018 ISO file. After the installation is complete, you can update to the October 2018 edition using Windows Update.
See Install Windows on your Mac with Boot Camp for instructions on installing Windows 10 using an image downloaded from Microsoft’s website.