Read the error message, it says all you need to know.
Since SFML.NET binds to the CSFML you need to copy the CSFML unmanaged dlls to the project output directory. The easiest way to do this is to add the CSFML dlls (csfml-window-2.dll, csfml-graphics-2.dll, csfml-audio-2.dll) to your project. You can find these in the "extlibs" folder. Then select the CSFML dlls in your project and set the "Copy to Output Directory" to "Copy if Newer".