I've been using Linux since 1996, so I have attempted numerous troubleshooting steps to solve this issue, albeit a relatively minor one. The computer in question has a fresh install of Ubuntu GNOME 16.04.1. Hardware specs do not seem to be relevant, but are available should you require them.
When I first installed the system, I used the GNOME "Software" app to install numerous programs (Stellarium, Inkscape, etc.) without issue. As soon as I would click "Install" I would be prompted to authenticate, and the app would immediately download and install. After installing numerous pieces of software (including things installed with "apt install" such as htop, tree, etc.), I went back to the "Software" app to install another program. This time, when clicking "Install", the status changes to "Pending" and I am never prompted to enter credentials. After several troubleshooting steps, including removing the "gnome-software" directory from
~/.cache, I decided to run "gnome-software" from the terminal. Oh, and yes, I have tried turning it off and back on again. :)
Initially, I was receiving the following error:
(gnome-software:3126): Gs-WARNING **: Failed to create permission org.freedesktop.packagekit.trigger-offline-update: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: Action org.freedesktop.packagekit.trigger-offline-update is not registered
Upon purging and reinstalling "gnome-software" (apt purge gnome-software, apt autoremove, apt install gnome-software") that error went away. Running it from the terminal now displays no errors or warnings, and immediately launches the app; however, the problem still persists. Any app I attempt to install gets stuck in "Pending" and I am never prompted for credentials.
I did find an Ask Ubuntu thread similar to this: gnome-software on Ubuntu 16.04 doesn't work
I've tried everything mentioned therein, but nothing seems to work.