The "Ubuntu Software" screen should be installed by default.
> Ubuntu Software Center is a one-stop shop for installing and removing software on your computer.
> It is included in Ubuntu 9.10 and later.
> - https://help.ubuntu.com/community/UbuntuSoftwareCenter
The "Ubuntu Software" screen should be installed by default.
"Should" being the operative word unfortunately. It clearly wasn't installed by default for this machine that had been upgraded through earlier versions (starting around 16 I think so well after 9.10), nor was there any obvious indication to the user that it was missing and available to be added.
There were some other oddities after that upgrade, for example Firefox no longer appearing for one-click launching from the default UI layout when it had before, so the lack of Ubuntu Software (and, apparently, its underlying apt package) wasn't the only anomaly. It just wasn't a polished experience that a non-technical user should have to deal with.
