It appears your duckling has swallowed something much too large. Whether there is a piece of debris stuck in her trachea, in her nares, or a large piece stuck in her esophagus putting internal pressure on her airways, unfortunately, it's unlikely we will be able to tell without a vet. The sneezing you're seeing, which I presume is consisting of a lot of head shaking is likely her trying to dislodge the debris.
You may try picking the duck up and checking inside her mouth, for anything visible that's lodged. Oftentimes you won't be able to see the obstruction, and without a vet, not much can be done. Ensuring the duck has a deep clean water dish may help in the removal of the piece since that's what waterfowl use to clean out their nares. For easier eating, you may consider moistening some of the feed and cease offering any large treats that may help contribute to her respiratory distress.