I am trying to create a server and add datapacks, so I downloaded my current world, added the datapacks in, and went to upload the world. It didn't work, and the message "The selected file/folder is not a valid world" showed up. I'm using Vanilla, and 1.15.2.
I then tried just downloading the world and reuploading it without touching it, but the same message appeared. Help? I've used the video linked from other related threads (https://youtu.be/01UzrsP589E.) Same issue. I don't see any reason why it's not a valid world - I literally downloaded and reuploaded the exact same file.