JExcel constantly gives you tips how you can use JExcel better in the form of messages.
Just watch the orange notification icon next to your workbook tabs and click on them (1).
Then you can see the problem messages and fix them with 1 click (2).
If there is a newer version of JExcel, another banner will help you in the top:
This banner can be disabled in the admin panel.
To turn it off, go to the icon (1), and select the option add-on (2).
Then select the tab "Configuration" (1), and select the option to hide it (2).