"Vegetation covers the surface of the planet."
Translation:yuQ ghor vel tI.
That's odd, but if so, I'm not sure whether there's anything we can do about it -- the error would presumably lie in Duolingo's scoring/correcting algorithm.
The next time this happens, it would be helpful if you could make a screenshot, upload it somewhere, and provide a link -- then we can try to pass on the bug report to Duolingo Central.
Or you could do so -- see https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug- .