Try downloading
viDrop and it should convert your videos fine. The thing with Rockchip based players is that you need to have the videos at a certain width x height resolution so it players, otherwise you will get "Format Error" messages.
Have a look at these posts as they pretty much as the same question and have the same player as yours:
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Rockchip video's!
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Cant convert AVI to play for MP4 IDream Touch Me 8Gb
Hope that helps.