"Das Mädchen lernt nicht."
If you have a Macbook hold 'alt' then press 'u' to bring up an umlaut, followed by the letter you wish to apply an umlaut to. For Windows you'll have to hold Alt then press a series of numbers depending which one you want. Google will have a list of numbers some where. Hope this helps.
You can just type 'ae' for a-umlaut, 'ue' for u-umlaut and 'oe' for o-umlaut. It is an accepted alternative to typing proper umlauts.
Just pointing out that by "accepted alternative" this means not only on Duolingo, but all across the German-speaking world. In fact the " actually used to be written as a small e over the letters.
You can configure your computer to switch keyboard layouts. On Windows, go to Control Panel -> Regions and Language -> Keyboard and Language
"did not learn" is past tense, but "lernt nicht" is present tense. So, "The girl is not learning" is correct (I think "does not" would work as well).