Install in thinkcentre Edge71 : LinuxCNC 2.9.4 Debian 12 Bookworm PREEMPT-RT ISO

Hi, I want to report my issues with this installation specific to the hardware in the title.. I'm learning on the go, not a computer guy so I can say some bull***.
Assuming this pc is legacy only I successfully installed LCNC (title) on reboot I'm getting error no operative system found. So I made my research and seems in my situation I need to install GRUB directly on the main partition ./root ? I somehow successfully installed it via the installer, but cannot replicate what I did previously for a new install, luckly I was taking some pictures of the previous screens that I guess installed GRUB manually. 

Somehow now that I decided to ask fot it I guess I finally succesfully installed it. using the autoinstall option, during the installation it asks if you want to force UEFI, at that point if you answer NO the auto-partitioner create just two partitions instead of three.

Then it finally asks you where you want to install the GRUB bootloader. where /dev/sda is the location to set