OK, I have now uninstalled the entire CB extension and reinstalled both the CB extension and the CBSubs plugin and still have the same result - not sure where to go now!
I have deleted the CB subs plugin and now Community Builder core works as expected. As soon as I attempt to add the CBSubs plugin I get the incomplete installation message as detailed in the attached image previously and the manage user option does not render properly showing no users registered (understandable if the CBSubs module is not functioning?)
I am currently running PHP 7.4 but changing this seems to make no difference and as previously stated version 3.9.24 of Joomla core.
In addition, I have now built a completely fresh test site using the same minimum components and can report exactly the same results. The main CB component works fine, however, when attempting to add the CBsubs plugin the install fails on the element: 002_cbplug_CBSubs_MAIN-4.5.0+build.2020.12.28.16.10.43.643177d9b.zip [Please see attached file]
Any guidance you feel able to offer would be greatly appreciated.