Seems something to ask with Google...
The language-neutral solution package was not found
Excerpt from the article:
There are several things you can try to resolve the issue, in this specific order:
Close Visual Studio Solution and completely exit out of it, then run it again and compile. This sometimes fixes the issue.
Right-Click on your Visual Studio Solution, select "Clean" Solution, then Right Click on SharePoint Project and choose "Retract". Rebuild and Deploy.
In PowerShell running as Administrator run Uninstall-SPSolution:
- Unistall-SPSolution -identity {Name of WSP File} -allwebapplications
In PowerShell running as Administrator run Remove-SPSolution:
- Remove-SPSolution -identity {Name of WSP File} -force
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!