I have a solution that worked for me, after some trial and error. Please bear with me as this is my first time answering a question on StackExchange.
I also had Org agenda break with the error message
Symbol's value as variable is void: org-tag-group-re. For me, this occurred while trying to install org-drill.
For context, I'm running Emacs 26.3 on MacOS Catalina, which was installed using homebrew.
Why it broke: The variable
org-tag-group-re is a regular expression that usually returns the tags (for example, :work: or :home:) associated with TODO items in agenda files. After investigation I found out that Emacs is using my system's version of org (9.1.9), rather than the most recent version of org (9.3.6 at time of writing).
org-tag-group-re is defined in the most recent version of org, but is not defined in the system version of org that Emacs is using, so it has the value
A solution: After reading about lisp libraries and messing around with my init file a bit, I got what may be a working solution. At the beginning of my init file, I added the most recent version of org to my load path and made sure that Emacs would preferentially load newer versions of packages (at Muihlinn's suggestion). The code for that looks like this:
(add-to-list 'load-path "~/.emacs.d/elpa/org*")
(setq load-prefer-newer t)
Notes: you may need to install the latest version of org if you don't have it already, and it may be located somewhere different. Also, I used the wildcard (*) to make sure that even if I upgrade to a future version of org, I won't have to change my init file. This is related to a similar problem on stackexchange that I saw while researching the problem, but I can't find it again. If you see it and comment the link, I'll add it and give due credit.