The -u- is the object prefix. It's not necessary, but it's optionally included for emphasis. In this badly designed course, there is one lesson where every sentence is supposed to have an object prefix, but both options need to be accepted throughout the course. In Duolingo, you don't always know what lesson a sentence is coming from, so all possible translations should be accepted.
Nitapanda mche. = I will plant a/the seedling.
Nitaupanda mche. I will plant the/a seedling. (Leaning slightly towards a translation with "the".)
Nitaupanda. = I will plant it. (where "it" refers to a seedling or anything really in the noun classes 3, 11 or 14)