Seems something to ask with Google...<br />
[The language-neutral solution package was not found](http://digioz.blogspot.com/2011/04/language-neutral-solution-package-was.html)

Excerpt from the article:

> There are several things you can try to resolve the issue, in this
> specific order:
> 
>  1. Close Visual Studio Solution and completely exit out of it, then run it again and compile. This sometimes fixes the issue.
>  
>  2. Right-Click on your Visual Studio Solution, select "Clean" Solution, then Right Click on SharePoint Project and choose "Retract".
> Rebuild and Deploy.
> 
>  3. In PowerShell running as Administrator run Uninstall-SPSolution:
>      - Unistall-SPSolution -identity {Name of WSP File} -allwebapplications
> 
>  4. In PowerShell running as Administrator run Remove-SPSolution:
>      - Remove-SPSolution -identity {Name of WSP File} -force
> 
>  5. In PowerShell running as Administrator run the Delete() command:
>      - (Get-SPSolution {Name of WSP File}).Delete()
> 
> 
> If items 1 through 4 doesn't fix it, item 5 will definitely do the
> trick, but it shouldn't be run unless all the other solutions fail.

**NOTE:** Before deleting anything just take backup!