I am running High Sierra 10.13.6 on my Mac pro 2013. When I try to run the update for this OS I keep getting this error: Update cannot be installed on this disk. This volume does not meet the requirements for this update

Are you running any VPN, Anti-Virus, or Cleaning apps?

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

https://pastebin.com/

Whew, they've changed pastebin & made it harder, but after pasting in, click Create new paste button, then Embed button, then copy the URL...

<script src="https://pastebin.com/embed_js/KuvnghqA"></script>

The important part is...

https://pastebin.com/embed_js/KuvnghqA

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...

Try installing the standalone update in Safe mode...

https://support.apple.com/kb/DL2046?locale=en_US

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode?

Which exact update is it?

Hmmm, how much free space is on the Drive?

Did it get other 10.13.6 Security Updates OK?