Error: file could not be opened successfully

I want to pack debian package with my app to snap. As a source (in parts section) I specified url to debian package file hosted on github. Source-type is set to deb.

It looks like this (it is a fragment without typical app information): confinement: classic parts: appName: plugin: dump source: urlToDebPackageHostedOnGithub source-type: deb apps: appName: command: usr/local/bin/appBinaryFile

It always throw error: file could not be opened successfully.