I was getting "The specified path does not contain a valid operating system or you do not have permission to access it. Specify a valid path." when I was attempting to add the image from \\CM01\Sources\OSD\OS\OSUpgradePackages\Windows10x64\1809.
The reason why I was getting the error is because the image file was install.esd. Apparently, the process requires a wim file, not a esd file. I used the following to convert the esd to wim. https://theitbros.com/convert-windows-esd-file-to-windows-wim-file/
After ensuring that the wim file was in the \\CM01\Sources\OSD\OS\OSUpgradePackages\Windows10x64\1809, the Add Operating System Upgrade Package Wizard completed.