If you see a fragment partition, select it and click the at the top of the screen to delete it and merge the space back into your main macOS partition.
Bugs in older macOS versions that interfere with the partitioning engine. Step 1: Run First Aid in Recovery Mode bootcamp 40 4033 windows 10 install
Type your Mac's login password when prompted (the letters won't show as you type) and hit Enter. If you see a fragment partition, select it
Here, you trick the system by manually creating space before opening Boot Camp Assistant. If you see a fragment partition
Select (Note: On older Macs, you may need to check "Create a Windows 7 or later version install disk" if you haven't prepared the USB yet).
A successful installation depends on thorough preparation. Do not skip these steps.