If you are missing the add-in in C/SIDE, then you get a message like this:
It is telling you that you have a variable that is referencing a DotNet component, which it cannot find in your local add-in folder. If it is in the folder it may have the wrong version number.
Normally, the Reports ForNAV setup program will copy the correct add-in to the client add-in folder during installation. This may fail if your client is not installed in the default location. In this case you need to copy the add-in to the client add-in folder manually. After installation of Reports ForNAV, you can find the add-in DLL file in this folder:
C:\Program Files\Reports ForNAV\Add-ins\ReportsForNAV\ForNav.Reports.dll
Copy this file to the add-in folder of your development client. Then this error should go away.