Deploy this solution to the same Web Application of the Site Collection where you are trying to activate this Feature.

This issue generally arises if the assemblies which are referenced in the solution are missing at virtual directory of the Web Application (Usually assemblies in 'Bin' folder of the IIS Web Application, C:\inetpub\wwwroot\wss\VirtualDirectories<port address>\bin)

-- Hope this might help someone out there as this post is quite old.