I see that error quite often, during system/OCLP installs.
It usually means that a reboot did not use the right partition sequence--so, the fix is to restart holding Option (so you get the boot selection screen). Choose the EFI partition, press enter, then manually select the correct next partition. Might be named "Installer", or simply the drive name that you have for the destination drive. It will (probably) already be selected, so you may be OK at that point to just press Return. The main step to NOT miss is that EFI partition first. I think the installer bypasses the EFI during a reboot, and it's important during the OCLP install. I have a couple of older Macs (2008 iMac, and 2010 iMac) that both have problems with that error. So, I wait for a boot chime announcing a reboot, and quickly hold down Option, just to make sure to boot through the EFI, then the actual boot drive that the installer wants. (the old 2008 iMac will not do a normal restart, so I have to always hold the power button until it shuts off, then Option Boot, all just to make sure the correct sequence is followed...
bottom line: Don't let your MP reboot, without making sure that the reboot sequence is there: EFI first, then whatever volume is selected to continue with the install. I call it "baby-sitting the installer"
