[SOLVED] "Failed to uninstall self. Please uninstall manually."

The Administrator Control Panel of my site is telling me that an update is available. I downloaded it and tried to install, but it's failing to install:

"Failed to uninstall self. Please uninstall manually."

When I expanded the error log, it showed me a 404 - Not Found message.

I uninstalled CB Package, then tried the installation again. Here's the log:

001_pkg_communitybuilder_2.1.2+build.2017.04.22.08.39.26.2c97ef2a0.zip is being installed.
001_pkg_communitybuilder_2.1.2+build.2017.04.22.08.39.26.2c97ef2a0.zip failed to install! hide
404 - Not Found
Installer is uninstalling.
Installer failed to uninstall! Error: 404 - Not Found

I had previously upgraded Joomla from 3.6.5 to 3.7 - should I have updated CB first, or is there something else wrong?
I'm not sure what the previous CB version was - the site is one that was abandoned by the original client and I've been maintaining it, so it was probably last updated a few months ago.

I have not upgraded all my sites to Joomla 3.7 - I'm testing sites that have been inactive first

Thank you,
Ernie

It occurred to me to check the ACL Manager, and there were TONS of missing assets and asset issues that needed to be corrected. Upon resolving them (ACL Manager is AWESOME!), I was able to successfully install the CB update.

I don't know when all these issues were created - I routinely "clean up" when I make updates on sites, but this issue stopped me before I completed all the work I was doing.

Maybe this will help others who encounter the issue.

Thank you,
Ernie