I do not know about System76 system.
They may just be using gpt partitioning with BIOS as that is how I boot my system which does not have UEFI capability.
But Windows only installs to gpt with UEFI. If you do not have UEFI, and you really want Windows you have to totally erase drive and start from scratch. You will have to use fixparts also as posted above.
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