Unable to install Security Update 2019-007 on my Mac running Sierra 10.13.6?

I believe you mean High Sierra?

Try it from here, this site uses a different server:

Download Security Update 2019-007 (High Sierra)

That might be a good thing.

Attempted to install the update last week on a MacBook Pro (17-inch, Mid 2010) with macOS High Sierra 10.13.6

During the process a window popped up alerting that the software, or was it operating system, could not be installed.

It then failed, multiple times, to reboot. : 0

Was, after much praying and a loss of a few more hairs, able to access an option to select the boot drive.

Thankfully the system is up and running again and a updated backup has been made.

Today the Software Update has reappeared. Think I will pass for now.

Anyone else with this issue?

Many have had the same issue.

Also, try installing it in safe mode...

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