No partition scheme option when erasing a USB disk in MacOS High Sierra?
I thought I had the same issue but it's probably because in the view options at the top left corner of the Disk Utility window, "Show Only Volumes" is selected. 

In order to change the partition map, it seems you have to select the actual disk. So simply select view - show all devices, then select the actual drive you're trying to reformat, then when trying to erase, you should have the option to choose a partition scheme. 

Hope this helps!