Duck is trying to play on a harmonica.
The painting was an order for someone who likes ducks and plays harmonica.
It symbolizes that it is good to have an aim in life. Also when it isn't always possible to reach. But not only the aim is important, also the way to get there. Try to enjoy as much as possible.
This comes back in the title of the painting: 'Allegro non Troppo'.
'Allegro' is an Italian music term that reflects the character of a piece of music. It means a lively, cheerful and living play.
'Allegro non Troppo' means lively, sparkling, but not to fast.