So I have finished a prototype map and was trying to get it on steam workshop using the publish function and this popped up.
"Error updating steam workshop item"

What I have done:
1. Map has no error and is playable.
2. I changed the loading and preview image of the map, and changed back to the original which still doesn't help.

Please help

I need to know too, I have no one idea what's wrong, can someone tell him and me? I would really like to update my map

just keep trying...

Look for a file called

workshop.json

in

zm_yourmapname\zone

and delete it, then retry uploading

Here are some standard conventions for solving this:
Clear your file (like mentioned above), compile/lights/link and see what errors occur (if there are any, it will not upload), if your preview image is not 512x512 it will not upload.