This means that the installation procedure has not been followed.

To correct this, you need to check your Add-ins :

Click on "..." then "Get add-ins". Or if you don't find it you can follow this link, it will get you to your add-ins on your Web version :

https://aka.ms/olksideload

Then go to my Add-ins :

If your installation is correct, the Add-in should be in "Managed by administrator".

If your add-in is incorrectly installed, you just need to click on the '...' then delete, and finally wait for the deployment to take place properly.

If your problem persists, don't hesitate to contact support via live chat.