RetroPie-manager has been removed since it's been un-maintained for years and it's not compatible with Python3. The original project repository has been deleted years ago and we kept a copy under RetroPie's Github account.
You can still find it at https://github.com/retropie/retropie-manager/ if you wish to try to make it work/install it.

@x20011I got that message the first time I tried to install desktop, pixel or the raspberry OS or whatever you want to call it, from retropie.
Fixed by re-burning the retopie image, Installing the Safe Shutdown and OSD Software on the PiBoy DMG.
After this I installed pixel and it worked.
But all this in my model 4.