Stacer installs from the software center but does not show up in the menus and does not run from the command line.
Tried to install Nomacs again. It installs - upon running it disappears from the menu bar and freezes the system.
To keep the system stable I had to remove both Stacer and Nomacs.
Personally I would recommend Ubuntu Cleaner over Stacer. It installs via PPA. You can read about it here.
http://ubuntuhandbook.org/index.php/2016/12/ubuntu-cleaner-ubuntu-tweak-alternative-janitor/
You can go to Preferences/Look and Feel/Main Menu/Properties/Command and see if Stacer is there. Check the command as they are not all the same as the name. Also uncheck and recheck and it may show up in your menu after that. You might also consider Bleachbit.
An example is Hardinfo, which says "System Profiler and Benchmark" on your menu, or Ubuntu cleaner, who's command is 'ubuntu-cleaner" or Main Menu is "mozo".
If you insist on Stacer, try this tutorial, I have found this guy one of the best on Ubuntu PPA tutorials on the net.
http://ubuntuhandbook.org/index.php/2019/06/stacer-system-optimizer-cleaner-monitor-ubuntu/
Sorry, I have no experience with Nomacs.