Without more information the best I could offer is to check permissions on the User and package share. Has anything changed recently?
In addition to checking the rights as Rick said:
Make sure the package is a package builder package.
Open the file up in package builder - make sure it is not corrupt.
Rebuild the package
Is the file accessed via http or unc?
Make sure you have the latest version of package builder
I'm now getting this error. Is there any new information on the fix?
I'm working with 8.7 SP 6
Ran across this with 9.6 SP2.
For me it seems to have been a corrupt network path. The .msi package failed even after recreating it, .exe packages also failed, when a batch file was attempted I got a ANSI/Unicode error. I renamed a folder and it solved the problem.