Boot Camp Assistant Issue

Hi everyone.

I have a MacBook Pro Retina 15 ( Mid 2014 ) and i have a problem about bootcamp.

When i was in Mavericks i installed Windows 7 on my Mac and then deleted it just because of free space.After that i updated my Mac to Yosemite and tried to install

windows once again but i couldnt.I had problem during bootcamp installation.Install or remove Windows 7 or later version section greyed out and couldn't check it.Then i updated my mac to El Capitan (latest version) and tried again but still same issue.What's the problem guys? I have to install windows for programming but i have no idea how to solve this.

Thanks!

*I have tried win7 win8 and win10 from Yosemite to El Capitan but doesnt work