I had the same problem and I’m sure just like me you have more than one disk drive.
The problem is about INSTALLING BOOT LOADER ON A NON GPT PARTITION DRIVE.
You have to change the installing boot loader drive OR change non GPT partition to GPT drive which will cause LOSING YOUR DATA.
I didn’t do the second solution because I had a windows 10 partition on that drive. Only problem for me is if I want to use my Windows 10, I have to manually remove the GPT drive so my system can load windows 10.
These are my solutions to my problem