SOLVED:
This may have been a coincidence but once I provisioned my unraid server in Management Access setting I was then able to download the file from GitHub.
I'm also getting the can't download error message. I'm not seeing anything that would prevent my unraid server from accessing GitHub. IPv4 DNS server is set to 1.1.1.1. As far as I can tell looking at the system log, when the pre_plugin_checks script is executed, I see "r8169 0000:02:00.0 eth0: Link is Down". Is this to be expected from the script or is this preventing the server from accessing GitHub?
plugin: installing: unraid-r8168.plg
Executing hook script: pre_plugin_checks
plugin: downloading: unraid-r8168.plg ... done
Executing hook script: pre_plugin_checks
-----------------------Downloading RTL8168 package!-------------------------
--------This could take some time, please don't close this window!----------
-----------------------Can't download RTL8168 package-----------------------
plugin: run failed: '/bin/bash' returned 1
Executing hook script: post_plugin_checks