Hello everyone,
I success last night.
In short, I believe if you played the world a bit far that explored quite far, you may not be able to download.
It probably related to the file size limited.
My main word is about 50MB and it is my first success. My last attempt was a world that I played for longer time with at least 4 big areas explored around 4 different portals in the main world. I think if it failed once, you will never success.
If you do not do anything, your "world" is called "world" in minehut.
The key commands:
/dl world
/dl world world_nether
/dl world world_the_end
Then you could refer to this link to learn about the folder structure to combine your zip files of main world, nether and the end.
https://minecraft.gamepedia.com/Tutorials/Map_downloads