I also found the below post which explain another cause.
So, basically, here is what can happen:
You are in the 3D Window Settings Dialog, switch to Internal Engine mode and change the Shaded Mode field to "Raster". Then you switch the 3D Engine field back to OpenGL.
The Shaded Mode field becomes greyed since it does not apply in case of OpenGL. However, it is still set to "Raster" and the result is that you cannot edit the Morph with the Pet Palette commands when you click a general face of the Morph. It still works if you click on Morph edges or nodes.
To bring back face editing commands, you have to switch to Internal Engine, set the Shaded Mode field back to "Analytic", then set the Engine back to OpenGL. Then suddenly you are able to edit Morph faces again:
http://archicad-talk.graphisoft.com/viewtopic.php?t=40009&postdays=0&postorder=asc&&start=10
I think this is a bug: ArchiCAD still takes into consideration the value of the Shading Mode field even when you use OpenGL. It should not, it should assume it is set to "Analytic".
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28