Q: Failed to install Windows 10

I am using iMac 27'' Retina 5K Late 2014, OS X El Capitan 10.11.4, on Boot Camp Windows 10.

Today I have damaged one software, which I need, so I have decided to reinstall. I did a backup of my data, rebooted to OS X, removed Windows.

Then I've downloaded Windows 10 ISO from Microsoft on Windows machine (because you can't get ISO directly, you need to run .exe), transferred to my iMac.

Using Boot Camp Assistant, I have made install USB for Windows 10. I have allocated about 300GB for Windows. After reboot to installer, I have formatted partition "BOOTCAMP". When I selected it to install Windows into, this somehow failed. The installer doesn't want to install Windows on that partition.

I have tried this three times, nothing has changed.

Does anyone have an idead, what I am doing wrong? When I was first installing Windows on Boot Camp, all went fine.