Hi there, for the error that occured on your Mac, you're recommended to erase the small partition with 16.8MB size and then install Windows one more time.
Firstly, you need to list all partitions on your device. Launch Terminal and run the command line.
diskutil list
Followingly, erase the partition on your device via Disk Utility.
Finally, Run SMC and NVRAM, and then try to install Windows on your macOS computer one more time. Do not connect any external hard drive to your Mac unless Boot Camp Assistant asks you to do so.