Fixed - Setup Was Unable to Use the Existing Partition
https://www.minitool.com/partition-disk/setup-cannot-use-system-partition.html

Receive an error message "Setup was unable to use the existing partition because system volume does not have required free space"

You can clearly see this 100MB system partition only has 0 free space left so that it does not contain the required free space

Dual Boot Mac OS X and Windows

To solve setup was unable to use the existing partition windows 10 Bootcamp issue, you can search on the internet to see whether there is partitioning tool for Mac to enlarge the system partition without data loss.

Disappointingly, so far we haven't found such a program. If you discover one, please share it with us so as to help more people. Much appreciate!
However, if you don't want to use third-party partitioning program, you'll need to delete both EFI partition and Mac OS partition, then reinstall Mac system without creating the EFI system partition or creating a larger EFI partition, and next install Windows to Bootcamp partition.

How to Download a Windows 10 ISO Without the Media Creation Tool

https://www.howtogeek.com/427223/how-to-download-a-windows-10-iso-without-the-media-creation-tool/

One exception is the HeiDoc.net Windows ISO Downloader tool, which lets you get the official Windows 10 ISO download from Microsoft. The program doesn't come from Microsoft but it does grab the ISO directly from Microsoft's website (software-download.microsoft.com), so unlike a torrent site or other file sharing method, it's most likely safe and can be a good alternative if Microsoft's tool (above) didn't work for you.