Other features or issues related to Microsoft Partner Center
Answer:
You can't edit your Store Listing directly unless you've first clicked "Start Update."
The UI is misleading because it lets you open the Store Listings page, edit the text, replace screenshots, and appears to allow changes. However, if your app isn't already in an Update state, clicking Save will fail without a clear explanation.
The correct workflow is:
Click Start Update.
Once the app enters the Update state, edit your Store Listing (including screenshots).
Save your changes.
Submit the update for review.
Without starting an update first, your Store Listing changes won't be accepted.