Can't check "install windows 7 or later" in BCA

I'm trying to install windows 10 on my macbook air (mid-2012) with macOS High Sierra using boot camp assistant.

I've downloaded the ISO disk image, but when I get to the select tasks part in BCA, the last option which is "install or remove Windows 7 or later version" is greyed out, therefore, I can't check it.

and when I click continue, I don't get the partitioning part in the window, but instead in the destination disk it chooses the backup hard disk and says that all data will be lost.

how do I enable that "install windows 7..." button?