Translation:She promised to write me every week.
This is another lesson where the contributors do not allow you to be even one letter off in a non gendered word. "premeteu" is what I wrote by accident. Sad, and that is definitely not teaching anyone anything. :( Not to mention, the lesson considered to be the opposite of this one, translation wise, does not allow "me" to be used, at all, in the least.
In the future, copy and paste your answer here, that way we can look at it and help you. As my comment says above, duo is often very particular about what it allows and what it does not. For example, if you are meant to say ela, but accidentally write ele, the sentence is now different, so they do not allow it. As is the tense for plural third person, where the verb escrevam is not the same as the verb escrevem, and writing the second instead of the first changes the context, and is therefore beyond a simple spelling error. Also, in this sentence, writing promete vs prometeu completely changes the context as well, and is this not a simple spelling error.
But, because you did not comply and paste your answer, which most of us mix up a letter from time to time, thinking we wrote it identically, it is hard to offer you any help. If it is the case that it was identical, then simply report it.