Namespace error from external assets

Hi everyone, english is not my first language so i'll try to explain you the problem as well as i can. I'm working on a project compiled from many people before me so i don't really know what did they do in it. This project contains a lot of assets imported from the asset store and, when i try to build it, every namespace relative to those assets give me the classic error "The type or namespace name `com' could not be found. Are you missing an assembly reference?" Every asset is in the Assets folder so I don't really know what to do to fix those 60 errors.