@Ongiegirl36 I'd get some liquid B complex [TSC] it will say injectable but we use it orally It has thiamine in it which is another vitamin your little one needs. Vit E and scrambled eggs for the selenium.You can mix a small amount of scrambled egg with the vit E and the B complex and feed it that way Or use a small dropper and give the Liquid B complex small drops at a time right inside the bill making sure she swallows with every drop. Since she is so tiny you can give her possibly 3 meals off one scrambled egg with the mixture in it.
Keep us updated.
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
this will give you doses for your duckling on the liquid B complex and how to administer it.