your currently installed windows 10 uses "GPT/EFI disk" partition structure, and that windows 7 which you are trying to install supports "MBR disk" partition structure, so you have to either convert disk (whole HDD) to MBR or make use of a UEFI/GPT based bootable windows 7,
I don,t actually know if windows 7 supports UEFI boot, as I have never seen UEFI in win 7, (although I might be wrong in my this win7 knowledge)
-or- just convert to MBR disk structure, there are many tools like easeus partition manager, etc (UEFI is modern whereas MBR is a Legacy system)
