To reproduce:
1. Open attached project and download the attached package
2. Open the script "Tester" in the project and make sure that the path is pointing to where the package "1" is saved on your machine
3. In the project menu, navigate to Test > Test - it tries to AssetDatabase.ImportPackage - but an error is thrown:
"Error while importing package: Package has unknown format"

Importing this package by drag&drop works fine. It seems that this due to the package being created on an older version of unity