Hi,
The .iso should be bootable. When we convert .img to .iso, it is not bootable. We can download Raspberry Pi Desktop for PC and Mac bootable installation media from the official website below.
https://downloads.raspberrypi.org/rpd_x86/images/rpd_x86-2021-01-12/2021-01-11-raspios-buster-i386.iso
For the detailed steps about installing Raspbian OS, you can refer to
https://www.avoiderrors.com/how-to-install-raspbian-os-on-hyper-v/
Note: the above two links are not from MS, just for your reference.
Regards,
Alex
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