Hej, can you really use both "... än jag" and "... an mig" in Swedish? Both is equally used? So in Swedish it is not wrong to use "... än mig"? And is "..än jag" used more often or the more regular form? In German it is wrong to say "niemand ist müder als mir" or even "... als mich" (which means both "me" in English and "mig" in Swedish). We can only say "niemand ist müder als ich." which is the exact translation of the sentence in Swedish. Tack!
Usually when that happens, you'll have had another mistake without realising. Duolingo is terrible at showing you the actually closest correct translation to what you wrote, unfortunately.
But it's also entirely possible that it's just missing, or that there is a bug. :)