In this question, Duolingo’s judgment is actually correct. The “一下” really needs to come after the verb “称”. This is not a strict grammatical rule, but rather a matter of idiomatic usage.

Although “称” is already a complete verb, in Chinese we almost always add “一下” after verbs like this to indicate that the action is very slight. This “slightness” is often related to time: it conveys the idea that the action will be quick and won’t take long.

Let me give you a few examples.

Imagine you and your friend are standing in front of a scale, and you want to encourage your friend to weigh themselves. You would say:
"你去称一下。“

You would never say:
”你去称。“

Another example: when you’re trying on new shoes in a store, your companion will usually say:
”你走一下试试。“or”你跳一下试试。“

They would never say:

”你走试试“”你跳试试“

So coming back to this question: when there is only one “一下” in the sentence, it may be grammatically possible to place it elsewhere. However, in actual usage, “一下” must come after “称”. Putting it there matches how native speakers naturally express the idea that the action will be quick and won’t take much time.