Installation, PC want's to mount a CD-ROM!!! Why and how please?

It can not be done without a USB or a DVD.
Well it can, but not easy at all.

Elco wrote:

I am running into the same problem on a Biostar mini-ITX board. But I boot from an SSD (loaded with the linuxcnc 2.8.2 - buster iso). 

Is that SSD meant to be the boot drive? The ISO is a bootable ISO image, but is really intended to be an installer and test environment. 

This issue with looking for a DVD is a problem with the Debian installer, and is only found on a subset of machines. You might find more answers looking on generic Debian forums. 
 

He has written the ISO image to SSD and is booting the PC from it and at the same time trying to install on that same SSD.

tommylight wrote:

He has written the ISO image to SSD and is booting the PC from it and at the same time trying to install on that same SSD.

I wonder if that can ever work? 

There is a way, use the empty space left from writing the ISO image to create another EXT4 partition, use that as root partition when installing and install GRUB on /dev/sda, not on /dev/sda1 or sda2.
Never actually tried this, but it should work.
Although the Debian installer is consistently inconsistent !