They are lacking in essential nutrients, probably niacin which can be why your duckling isn't standing or walking niacin deficiency cause leg problems. They need a balanced diet which only a complete feed will give them especially when at this age and all the nutrients are being used to grow strong legs and healthy body. Can you find a store that sells chick or duckling feed? how are chicks raised in Korea? surely there is chick feed which would be much better for your ducklings and then add a vitamin to give them the extra niacin they need till around 12 weeks old. Do you give them a bowl deep enough for them to dunk their heads into so they can keep their eyes and nares washed out? this is very important also for ducks. Here is a link that will help you better understand the basics of duckling care. I hope your ducklings begin to get over this set back, I think if you can find some good feed that will be a start.
https://www.backyardchickens.com/t/750869/raising-and-caring-for-ducklings#post_10611711
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Agreed.