Are you the (un)lucky owner of a modern HP printer which needs a closed-source binary plug-in? And are you using Ubuntu Linux? Then you might know the detailed error message Plug-in install failed which occurs when you try to setup the printer with HP Toolbox (From the hplip-gui package shipped with Ubuntu):
Looks like there is a permission problem somewhere. Not sure if it is Ubuntu's or HP's fault. Running the HP Toolbox as root might fix the problem but unfortunately it refuses to run as root. But there is an easy way to work around this problem. When you try to install the plug-in the HP Toolbox downloads a plug-in installation script from an HP server to your
/tmp directory. It then executes it (Without root privileges I guess) which fails. That's why you get the Plug-in install failed error message. Cancel all dialogs and close the HP Toolbox and execute this command in your terminal:
sudo bash /tmp/hplip-3.12.2-plugin.run.1
The version number may be different so check out which files are located in your
/tmp directory first. You'll have to confirm the license and after the installation is complete it will print Done to the console. When it didn't print Done then something went wrong. Maybe you forgot to execute it with
sudo? When the installation was successful then start the HP Toolbox again and setup your printer. The tool will not try to install the plug-in again because this time it is already there and it works.
Happy printing (and happy scanning and faxing if your printer supports it)!