Translation:His speech is not lacking in dignity.
When the sentence is negative the article disappears.
- ne manque pas/plus/jamais de dignité.
It would come back if the noun were specific, like:
- ne manque pas de la dignité qu'on en attendait = is not lacking in the dignity we expected
must it be "is not lacking "? my answer was "her speech does not miss (or lack in) the dignity" and refused.