Win10 unable to instal on iMac late 2015 with El Capitan

I have late 2015 iMac 5K with El Capitan updated. Tried installing Win10 from an ISO file downloaded from Microsoft site.

Launched BootCamp. It did not ask for a USB flash drive or give any other option. It went straight to ISO image selection and then disc partition.

Disc was partitioned and WIN10 setup started. It then asked to choose where to instal the Windows. I choose the portion created by the BootCamp. This portion was formatted. When I clicked on instal, I got error message"UNABLE TO LOCATE THE PARTITION OR CREATE ONE" and the installation could not proceed further and had to be aborted.

Would greatly appreciate help in solving the prob;em