I had Steam installed and working fine on Ubuntu 15.04 x64.
I upgraded to 15.10, and Steam stopped working.
Because Steam was having trouble with the mesa-dri:i386 drivers, I uninstalled Steam with synaptic and downloaded and installed Steam.deb directly from Valve. That produced the same error message, but will launch the Steam client.
I then uninstalled that again.
When I do:
sudo apt-get install steam
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
steam:i386 : Depends: libgl1-mesa-dri:i386 but it is not going to be installed
Depends: libgl1-mesa-glx:i386 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
I have read many posts that address this problem mostly on other version of Ubuntu, none of those solutions have solved my situation.