I'm running macOS Sierra on an early 2015 MacBook Pro with a 500 GB SSD.
I've been trying to get Windows 10 installed via Bootcamp by following this guide: https://support.apple.com/en-us/HT204990
I've allocated 80GB of space via the Bootcamp Assistant.
I'm getting stuck at the following error:
"Setup was unable to use the existing system partition because it does not contain the required free space"
I can go larger, but am not confident that would resolve the issue (I've done some searching and others report an issue with 100GB) and I didn't really want to give Windows that much space.