This error can have multiple meanings depending on the error message:
File doesn't contain importable data
The file selected to import doesn't have any importable information. It may be an empty file or be the wrong type of file. Check the file contents to verify that it is correct.
Library package file not valid
The Microsoft Authenticode signature of the package downloaded from the Package Library cannot be validated. This could mean that the root certificates used by Authenticode have been removed or corrupted. See this Microsoft Knowledge Base article. Otherwise, submit the error to email@example.com to examine for the root cause of the error.