As you can read in the title..
$ sudo apt install steam
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package steam:i386 is not available, but is referred to by another
This may mean that the package is missing, has been obsoleted, or is only available from another source
E: Package 'steam:i386' has no installation candidate
On a sidenote, I ALWAYS get errors on Ubuntu nowadays when I try to install stuff. I can't install through the Software Center either - nothing happens when I'm clicking the Install button!