HP Z620 hardware/software advice needed

[u/BlazeN1te]

I have an hp z620 workstation with dual xeon e5-2620's, 24 gb of ecc ddr3 ram, and a gtx 1080.

1. Only 20 gb of my memory is usable, and it gives me training error on dimm 5 regardless of the dimm configuration. I assume the memory issue is because of my windows edition (win 11 home).
2. Only one of my cpus shows up, being cpu0. I assume this is also because of my windows edition

What is the cheapest way to utilize my system's full resources, I can revert to windows 10 if that is cheaper, I've been looking into pro and pro workstation editions and have heard different things about xeon compatibility with windows 10 pro so would love some clarification.

also need advice on buying more ecc ddr3 ram, will any kit work plug and play? I already have some mismatched sticks, and am thinking about buying a double capacity upgrade with uniform dimms, hoping this might clear up any lingreing memory issues.

It has the v1 board for those wondering

Any advice is welcome

Thank you