Unable to install an update via the softwareupdate command line tool
As documented in the man page you need to use sudo to actually install software via softwareupdate. So in your case it's
sudo /usr/sbin/softwareupdate -i &quot;macOS Catalina 10.15.7 Update&quot;

to install 10.15.7, or
sudo /usr/sbin/softwareupdate -ia

to install all available updates.