Restart the game, go to world editor and make a brand new map from scratch. Put a little bit of terrain, some trees and some player spawns, put a random name and desciprion then try the Steam tab again to see if it lets you publish a brand new map on the workshop.
If it does, we'll know the issue is with the map you're trying to upload currently.
Comments
Have you considered also sharing it on the Steam workshop?
If it does, we'll know the issue is with the map you're trying to upload currently.
Smaller mini map uploaded Correctly
other map must have a problem?