I ran into a similar issue when attempting to dual boot Ubuntu 18.04 on a machine running Windows 10 with legacy BIOS. During the Ubuntu install process, the installer did not recognize the Windows 10 OS.
diskmgmt.exe in Windows and saw that the disk had an MBR partition:
Previously, I had Windows 7 installed and got upgraded to Windows 10. I think this is how I ended up getting Windows 10 with an MBR partition. From my understanding, Windows 10 needs to have a GPT partition in order to have UEFI BIOS, otherwise you'll end up with legacy BIOS.
This seems to cause the error during the install process. Ubuntu 18.04 has a UEFI BIOS; Windows 10 will also need to have a UEFI BIOS, or it won't be recognized.
I ran the
mbr2gpt tool that comes installed with Windows 10 Creator's Edition:
This procedure will allow you to change the Windows partition from MBR to GPT. You'll need to get a WindowsPE USB to run this tool:
After changing the partition, you'll be able to switch to UEFI mode in your BIOS. Once the BIOS was set to UEFI, the Ubuntu installer recognized the Windows 10 OS and I was able to dual boot both systems.