No packages were eligible for install
The accepted answer to the question (How can I download an older version of OS X/macOS?) that @nohillside suggested contained the ultimate solution--essentially the computer's date needed to be within a year of the OS' release.

Wikipedia's article on OS X El Capitan indicated that:


  OS X El Capitan was released to end users on September 30, 2015, as a free upgrade through the Mac App Store.


Based on this knowledge, I changed the computer's system date to 2/1/2016 12:00:00 @ UTC using Terminal:

$ date -u 020100002016


Once the system's date was changed, the installation completed as desired.