Q: problem with installing win10 on mac el captain

This is usually due to unavailable contiguous disk space. You may want to boot into Internet Recovery and run

diskutil repairDisk disk0

Can you try a wired connection instead of a wireless connection and use Command+Opt+R to try and get to Internet Recovery?

Do you have a Time Machine Backup setup for your iMac?

1. Backup OSX and all your files - Use Time Machine to back up or restore your Mac - Apple Support .

2. Boot into Internet Recovery (Command+Opt+R) - OS X: About OS X Recovery - Apple Support .

3. Click on Utilties -> Disk Utility and Erase your internal whole disk.

4. Restore OSX and your files - Use Time Machine to back up or restore your Mac - Apple Support .

This requires a separate external disk which can accommodate TM backup - Backup disks you can use with Time Machine - Apple Support .

Once you have OS X restored, please retry BC Assistant.

Can you check if you can reach the servers listed in Requirements for Software Update Service - Apple Support ? This error you see usually indicates network issues.

Do not try HTTP(s). Try ping or traceroute.

Start Applications -> Utilities -> Terminal, and copy/paste each individual line in the Terminal

ping -c 3 swscan.apple.com

ping -c 3 swquery.apple.com

ping -c 3 swdownload.apple.com

ping -c 3 swcdn.apple.com