At the very start of the installation process, press Shift + F10, or Shift + Fn + F10 to open Command Prompt
Run this command and hit Enter:
diskpart
Run this command and hit Enter:
list disk
(Your need to be very careful with this step, make note of the disk number you want to convert and enter it into the next command - replace X)
Run this command and hit Enter:
select disk X
Run this command and hit Enter:
clean
Run this command and hit Enter:
convert gpt
Then close Command Prompt
Then start the installation process again.