Setup asks for Disk #12 but the ISO only contains a CD. Solution: You are booting a floppy-based ISO. You need to switch to a true CD-ROM ISO. Look for files named NT31CD.iso not NT31FLOPPY.iso .
Let's address the elephant in the room:
NT 3.1 often requires a "Boot Disk" (floppy image) even if using a CD. You may need to mount disk01.img alongside the ISO. windows nt 3.1 iso
By creating an ISO, modern users are retrofitting the past into a usable container. It is an act of digital necromancy. We want to drag NT 3.1’s monolithic kernel, its stark blue login screen, and its chunky three-dimensional file manager into a world of SSDs and cloud storage. Setup asks for Disk #12 but the ISO only contains a CD