Minecraft doesn't let you back up your incompatible world even though it says to. (Reupload, due to Resolution: Incomplete)

When I tried to load up a world from a different version, it says "Make sure you backup this world before you load it in this version". I tried backing it up in the original version that it was created, but that didn't work, and it still told me to backup that world. It still says that it's incompatible.