The player spawn has to be on or right above a surface you can stand on, if it's inside the ground it won't work, and it won't work if it's too far off the ground either.

Fix that, everything should be good!

Once you hit upload the game wants you to complete your level once, then you get to take a screenshot of it for on the workshop afterwards and it'll start uploading.

thanks that was the problem, would be nice to have some sort of popup informing thats the problem