Installing Windows 10 "Windows can't be installed on MBR. Convert to GPT"

Yes don't do that! Clean will quite literally clean/format the HDD/SSD.

There isn't really a way to convert from MBR to GPT without destroying the disks contents though as far as I'm aware (unless times have changed?).

The only advise I could give is back everything up and flatten the HDD and install Windows 10 fresh.

... Though Windows 10 /shouldn't/ require GPT, are you sure you didn't boot it up as a UEFI install?

For people configuring their PCs for the first time all by themselves, an error message like “Windows cannot be installed on this Disk” can be a big time problem. One, the message explains little about its cause and second, most of us don’t really have a lot of experience in setting up systems. The problem, however, is not as elephantine as it seems. It is simply a compatibility issue. This problem generally occurs when you try to install windows on a UEFI-based system while your hard drive partition scheme does not support UEFI.

The agreement between the firmware and the hard disk partition scheme is essential for locating the boot loader for the Operating System and hence, is the first step in the installation process of windows on a computer system. By default, recent hardware boot using UEFI mode. Thus, when one tries to use an old hard disk with an updated UEFI-based system, the message “Windows Cannot be Installed On this device” pops up. This is because old hard drives maintain partitions using MBR scheme which is not supported in UEFI-based systems. Therefore, for windows installation it is important to convert either MBR partition scheme to GPT partition scheme or configure the system to boot with the legacy BIOS-compatibility mode.

To solve this problem without data loss, you need to convert MBR to GPT disk with third party software or enable Legacy boot mode in BIOS.