I figured out what was causing the issue.
I had a folder for tapatalk in the /src/addons folder that I had renamed to tapatalk_old and then uploaded the newest version of tapatalk. These folders were conflicting with one another. I removed the old folder and the error went away.
I updated to the newest version (which was a minor upgrade). Afterwards, I see the following on the Admin Home screen and the Add-On screen:
InvalidArgumentException: Must provide an existing add-on or add-on JSON in src/XF/AddOn/Manager.php at line 270
XF\AddOn\Manager->loadAddOnClass() in...