Installer fails

I’m following the instructions at RockpiX/install/Windows.iso - Radxa
everything is fine until I poweron to boot. Press del during the bios boot, and select the
USB stick with UEFI boot partition. After a short pause the Windows logo pops up, then quickly disappears and I get a screen on the HDMI which appears to be the built in UEFI boot screen
with yellow text and a Shell>> prompt.

It also does this same thing installing Ubuntu.
any ideas?

In this case your Installation USB stick is not bootable or you did not select and saved it right in BIOS settings.
But if you are on that prompt you described just type exit and press enter. If there is any other available bootable device it should try to boot from that device(s).