You might need to update your macOS or use a later edition of Windows 10.
In certain circumstances, Boot Camp Assistant might say that your bootable USB drive could not be created, or that there is not enough space available on the disk. To resolve, complete these steps:
Information about products not manufactured by Apple, or independent websites not controlled or tested by Apple, is provided without recommendation or endorsement. Apple assumes no responsibility with regard to the selection, performance, or use of third-party websites or products. Apple makes no representations regarding third-party website accuracy or reliability. Contact the vendor for additional information.