Translation:You only need to pay five percent interest.
"You need only pay five percent in interest" is a perfectly acceptable way to speak in English. Why is it considered wrong? Same meaning, proper grammar…
The preposition in "percent in interest" isn't really used, as far as I'm aware. For instance, "percent in interest" yields 98 results on Google, while "percent interest" gives over half a million hits.
I concede that one expression is more common, but Lisa939448 is right. Her translation is perfectly acceptable as an English sentence (especially because I translated the sentence the same way). Could Duo be a bit more flexible since the object under consideration here is learning Swedish, not debating which forms of English diction are more popular?
"Undergraduate students can expect to pay 4.45 percent in interest on new Stafford loans, instead of the current 3.76 percent."--Danielle Douglas-Gabriel, "Say Goodbye to Low Interest Rates on Student Loans, at least for Now," Washington Post, May 11, 2017.
I still think "You need only pay five percent interest" should be acceptable.