The default settings for the iThemes Security plugin (which I’ve recommended here) are such as to prevent the statistics gathering plugin Matomo (which I also recommend) from working. The symptom of the problem is that you get a 403 Forbidden (or Not Allowed) error when trying to view the Matomo reporting screen. Matomo is also failing to record visitor statistics, but you can’t tell that’s happening because you can’t see the reporting screen. 🙂
To use Matomo while iThemes Security is active, go into the iThemes Security > Settings screen.
Click Advanced, and uncheck “Disable PHP in Plugins”.
This will fix both problems — it will allow Matomo to gather statistics, and also will let you view those statistics.