you probably need to boot from USB to do this
If WIn 10
On another PC, download the
Windows 10 media creation tool and use it to make a win 10 installer on USB
If win 11
either download the media creation tool (2nd link) or download the ISO (3rd link) and use it to create a bootable USB -
https://www.microsoft.com/en-au/software-download/windows11
Now once you have an installer,
Boot from USB
on screen after languages, choose repair this pc, not install.
choose troubleshoot
choose advanced
choose command prompt
type:
bcdedit /deletevalue {default} truncatememory
press enter
Then exit and restart
that should fix your memory problem.
as for the 8gb, did you check the ram sticks as windows 10/11 are more likely to...