This post is for anyone else that had to delay upgrading to 2020.2.4 for any reason, you may run in to the same issue.
When trying to update from 2020.2 to 2020.2.4, during the installation, it failed at installing NCM saying "The installation source for this product is not available. Verify that the source exists and that you can access it". After checking, the file it was looking for NCM-2020.2.0.6162-NcmInstaller.msi was indeed missing. I guess the 2020.2.4 installer needed to see the old installer file as well.
After checking Windows logs, I found out that because we had updated virus definitions, our antivirus actually deleted the file because it was labeled as a trojan (and was also preventing me from manually placing the file in the Solarwinds\Installers folder). I had to override the security control for that file to allow me to place it from the old 2020.2 install\SubInstallers. Once placed there, the installer went through just fine.
Hope this helps.
