Hi Parveen,
this indicates that the manifest.webapp file is missing or misplaced. The manifest file should be placed in the root of the archive (Zip file).
A common error is to create the Zip archive off an outer directory. You should create the Zip archive so that the root directory of the app (the one containing the manifest file) is the root of the archive.
Example: Have a folder structure similar to this:
/js
/img
/docs
/manifest.webapp
Create the Zip archive with this level as the archive root.
Docs here: Home - DHIS2 Documentation