This is a very rare error, but has happened to me once before while setting up the local inventory ads for a client.
If your store is closed, ie not open yet. The supplement feed will show error Invalid Store Code. To resolve this, make sure your store is open.
If the above reason is not your case, after some back and forth with Google, they suggest to bulk upload the store information in Google My Business, and this resolved my issue.
Do not use the “Add another business” option.