If it is working in the IDE, the variable mapping is ok. This is a permissions issue, or a relative folder issue.

Check permissions first - log in with the SQL Agent service account and try running the package. In the default configurations, the packages being run through the Job Agent will run with the SQL Agent credentials.

Remember that the folders the package can see are relative to the server the Job Agent is running on - not the server the SSIS package is stored on (if that is a different server). So, make sure the folders are on the correct drive relative to the server that is running the package. Also remember that the SQL Agent is running as a service so mapped drives may not be there - use UNC for network drives.