By some strange reason, installing Steam directly from the Software Centre doesn't always work, and I have often had to look it up. Someone needs to put in the logic to add i386 repositories and enable the partner repositories before trying to install Steam from the Software Centre.
There are lots of different advice out there and not all of it actually works, so I thought I would write a blog post to remember how to do it and possibly help others with the same problem. This worked for me in Ubuntu 19.10 and Ubuntu 18.04 and it will probably work in future versions like Ubuntu 20.04 as well.
For any of this to work, you have to have enabled repositories from Cannonical partners in your Software Sources control panel since Steam is in those.
Then open a terminal and execute the following:
sudo dpkg –add-architecture i386
sudo apt update
sudo apt install steam-installer
The first command adds the i386 architecture to your repositories. This is important because most Steam games are i386, not AMD_64, so you need to get the i386 versions of libraries for things to work properly.
The second command updates apt's list of packages available. You should always do this after adding an architecture or a repository.
The third command installs the steam-installer. When you run this later on, it downloads the newest update to the Steam runtime and installs Steam properly.
Since Steam uses the Microsoft Arial font a lot, you might also want to install the Microsoft fonts with this command:
sudo apt install ttf-mscorefonts-installer
Another option is to not just install the Microsoft fonts, but all of the Ubuntu restricted extras for better DVD-reading/ripping support and such:
sudo apt install ubuntu-restricted-extras