"You wrote it wrong."
Up until writing this sentence wrong (ironic ), I was under the impression that it was okay to place 了 immediately after the verb when indicating action completion. In fact, I'm pretty sure that every quiz question I've encountered on Duolingo up until this point has accepted English-to-Chinese translations utilizing 了 in this way. Unfortunately, as it turns out, sentences must satisfy one of several criteria for this pattern to be permissible. What those criteria are can be found here: https://resources.allsetlearning.com/chinese/grammar/Expressing_completion_with_%22le%22#Putting_.E4.BA.86_After_a_Verb_with_an_Object