PC specification:
PC name: Acer Veriton M6610G from 2010
CPU: Intel Core i5-2500 (stock) (4c/4t)
GPU: GeForce GTX 460 with 1GB of VRAM (stock)
RAM: 8GB DDR3
Boot drive: DVD SATA drive
Booting with “compatibility” setting in the BIOS (legacy boot)
Not exactly. Browsing the code, the message is shown when the boot volume is not of the “package management” type. The issue seems to be that booting form CD (and, I think, floppy, network, anything the BIOS does not treat as a HD) is different and you get a TAR filesystem that does not have packages or does not recognize them, I don’t really know the details.
It may be different for UEFI, if you have the choice in your firmware.
Just tried it. Indeed, this option is only disabled in BIOS mode. When you’re booting on same hardware from same DVD but in UEFI, it becomes available.
Unfortunately, it doesn’t seem an option for OP.