BootCamp The startup disk does not have enough space to be partitioned
The free space required by the Boot Camp Assistant to install Windows is larger than what is actually required. One question where this is documented is Mojave bootcamp assistant limits partition size. The answer to this question was to use the Disk Utility to create a partition for Windows, then install Windows the same way as any ordinary PC owner. The Boot Camp Assistant was only used to download the Windows Support Software. 

Other space issues related to installing Windows are documented in the accepted answer to the question: Installing Windows on MBP 2015 (Mojave) Using Boot Camp, Error: Not enough space!.