appstream-util version 0.7.16 complains:

  $ appstream-util validate-relax --nonet ./data/sm.puri.Calls.appdata.xml
  ./data/sm.puri.Calls.appdata.xml: FAILED:
  • tag-missing           : <description> required
  Validation of files failed

What it actually means that the <description> is not well formed because
it should contain a paragraph.