Error that refers to Autodesk.RevitUI.IExternalCommandM

Hi all!
I refer to the library with extension .dll C#. what could it be?
button error occurs:

The add-in failed to initialize because a class was not found in the add-in assembly… The FullClassName value specifies the entry point at which the add-in application is called from Revit. To run the add-in using Revit, the “Autodesk.RevitUI.IExternalCommandM” interface must be implemented for this class.

I did as in the help.
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