The two possibilities that I see based on the error message are:
- Emacs was not compiled with png support
- You do not have a program to convert from off to png installed
The ways to check each of these are:
- Try to open a png in Emacs if you see a picture in image mode (you may need to hit
C-c C-c or something) then you have png support.
- Open a terminal and run
gs. if this program is installed then you should be good. Otherwise, install Ghostscript
If you don't have png support them I don't really know what you can do easily. You may be able to find some distro specific instructions for a more complete installation or you may have to build Emacs yourself.
I should probably add that if you get docview working then you should look into pdftools as it can give you some more advanced pdf-specific features