Unable to install an update via the softwareupdate command line tool

I am trying to install a macOS update from the command line. Let's see what updates are available:

$/usr/sbin/softwareupdate -l
Software Update Tool

Finding available software
Software Update found the following new or updated software:
* Label: macOS Catalina 10.15.7 Update- 
    Title: macOS Catalina 10.15.7 Update, Version:  , Size: 5099042K, Recommended: YES, Action: restart, 

But what is the actual label? According to this resource https://www.macrumors.com/how-to/update-macos-terminal-command/ it is the exact wording:

So the analogous value here would be macOS Catalina 10.15.7 Update- . However that does not work:

 $/usr/sbin/softwareupdate -i "macOS Catalina 10.15.7 Update-"
Software Update Tool

macOS Catalina 10.15.7 Update-: No such update
No updates are available.

I also tried some variants:

(python39) 10:25:21/~ $/usr/sbin/softwareupdate -i "macOS Catalina 10.15.7 Update"
Software Update Tool

macOS Catalina 10.15.7 Update: No such update
No updates are available.

$/usr/sbin/softwareupdate -i "macOS Catalina 10.15.7"
Software Update Tool

macOS Catalina 10.15.7: No such update
No updates are available.

So then how is the softwareupdate tool supposed to be used ?

As of Sept 2021, when attempting an upgrade to Catalina 10.15.7...

I ran into the same problems as OP but was forced to modify my solution from accepted answer.

No amount of the following made a difference on their own. The result was always the same.

• sudo
• /usr/sbin
• Label v Title name

The Fix

This is the line that eventually worked for me: sudo /usr/sbin/softwareupdate -ia 'macOS Catalina 10.15.7 Update'

Breakdown of CLI

• sudo I threw this in even though I'm running as system. It worked.
• /usr/sbin/softwareupdate proper shell pathing should void the need to add the path, but multiple forums suggested otherwise. So, it didn't hurt to add and it worked
• -ia ... WTF? -i is for install... so that's good. -a is for all available... see below
• 'this is the actual Title of the update, not the label': To get this, I run softwareupdate -l | grep Title. Then I pull out the title from the other info

This is Stupid & Broken

Yes. The Fix worked. But... why was it necessary to use the -a flag? That's expressly saved for installing all available updates. However, when running this command, it only installed the targeted update and left the BigSur update uninstalled.

Apple has clearly broken something in the tool, and it seems with the impending MDM restrictions and deprecation in macOS Big Sur, has no intention of fixing this.

I hope this works for you before Apple sunsets remote administration completely.

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 "macOS Catalina 10.15.7 Update"

to install 10.15.7, or

sudo /usr/sbin/softwareupdate -ia

to install all available updates.