Error occurred in deployment step 'Retract Solution': The language-neutral solution package was not found.
The chance is this solution wasn't properly retracted, and SharePoint still thinks it is installed. A simple way to fix is to change solution GUID. The GUID and wsp name is defined in the Package\Package.package, but you need to open this file in text editor such as Notepad (not in Visual Studio), and restart Visual Studio.
