I installed it knowing it had not been tested on my version; it says so on the description page. So, I'm testing with hopes of using the plug-in.
Here's what happened: Plug-in installed and had directed me to the setup page; I made some changes and clicked "Update" button. The plugin then said I didn't have permission to access the page (hmmm, the same account/user installed it). The "Web-Invoice" tab (on the left-side navigation WP-admin area) disappeared after that.
I've since deactivated the plug-in and tried again with same result.