Hi Nicole,

for me it looks like old Helix plugin, which you have left from the past, from old template maybe.

1. Switch to PHP 7.4
2. in Admin - On the list of plugins find "helix" or "shortcodes" plugin and disable it.
3. But keep all "Helix3" named plugins enabled.

Altenative "emergency" way that may (or not) help:

clean content of that file, just make it empty inside: plugins/system/helix/core/wp_shortcodes.php

I was able to upgrade but now on the front end of things I'm getting an error, 404 Class "JEventDispatcher" not found

Probably Your error is still from old code from plugin or module ...

and in meantime, I will check as well. Hopefully admin is working ;)

look down ^-^

Ok, Fixed & found source of problem - that extra installed module made the mess (error)

I disabled it, now decide what to do with them. Your turn.

I think you should find alternative module that works in Joomla 4.x and use it. And currect -> trash/uninstall.