This thread has tracked the problem for the last few years.
If you are not familiar with this important system family, it allows you to create curtain wall screens, (no glass between mullions) such as the one below.
However, whilst these two screen appear the same, only one of them can manipulate it's shape or attach to slope roofs.
You see, to make things nice and confusing, there is an 'Empty Panel' loadable family available in the default library that comes with your Revit installation. However, it is a LOADABLE family, not a SYSTEM family. The system family comes with special powers that allows its shape to be manipulated. Anyone who has ever tried to attach a curtain wall with custom panels to a slope roof before, or has come across this empty panel problem, will likely have encountered this error message:
The beauty of the system Solid, Glazed & Empty panels, is they allow for this manipulation without throwing up this warning.
Finally, being a system family, it isn't loadable, just like walls, floors, roofs and ceilings to name a few. Therefore, the only way to get this into your project should you need it, is to copy and paste a curtain wall that uses the empty system panel family into your project. For some, this may not be so easy if you don't have a copy of a pre 2017 template lying around.
I by chance had one on file and have saved this to a blank template which now lives in one of my cloud storage drives so I will ALWAYS have it on hand
Hopefully Autodesk take my problem report I logged today seriously enough to fix the next release template.