It's Windows 7 Ultimate SP1 x64. But it's not an OS-dependent issue anyway. It should be something about BIOS. It limits the usable RAM space to 64GB and likely needs to be unlocked somehow to get the 128GB mapped (and it's capable of doing so).
Some older BIOS cannot address more than a certain amount of RAM. Might also be a limitation of the chipset (Northbridge AFAIK). Sometimes it works with enabled Memory Hole Remapping.
I don't think this machine was meant to go beyond 64GB of total ram capacity. Everywhere I look it says "max 64GB", even Crucial's ram upgrade website only gives you 2x 32GB as your top option.
This PC is absolutely capable of using up to 128GB RAM. But in order to achieve this, BIOS needs to be set / modified / 128GB-unlocked. I've seen community posts where people reported having Clevo P751TM1 / P775TM1 with 128GB RAM, as well as the info that BIOS needs to be modified / 128GB-unlocked somehow