From Toshiba Link Removed
I have no idea if that reference information is pertinent or not, I just thought I would pass it along.
Seems counter intuitive if you ask me. I can understand disabling secure boot and maybe fast boot too, but it seems that if you disable UEFI and use legacy mode then you're going to end up where you are now, with a message box telling you that you have a GPT (guid partition table) and you can't install there even though you've created a UEFI / GPT thumb drive of the installation media, you've turned off UEFI so no dice on the GPT.
I think you need to maybe submit a trouble ticket with Toshiba or perhaps maybe if they have a support forum maybe someone can make some suggestions.