I find that very difficult to even comprehend.

Mail merge works with the current release version of Thunderbird, there is even a version on the web site for version 68 that has not even been released officially yet and old version support appears to go back to Thunderbird Version2.

Have you looked in the add-on manager to see if it is awaiting a restart of Thunderbird to complete the upgrade of the add-on to whatever new version you have now?

I have Thunderbird version 69.ob2 mail merge doesn't show up in the add-on manager.

69 is about as bleeding edge as you can get. It is also a product that is in very early beta testing, it is not a release version and will not only lack add-ons, but will be buggy. Were you intending to be a beta tester? If not in your case reinstall using the release version at getthunderbird.com