Hi Elen,
Can you verify that the "repo_name" *exactly* matches the configured repository name in Review Board? This is case-sensitive.
Make sure the repository is marked as Visible and that there's either no access list defined or the access list includes the user you're logged in as in RBTools.
If that all seems correct, would you be able to provide a full debug log (rbt post -d <changenum>) and a screenshot of the repository settings in Review Board? (You can redact the Path/Mirror Path fields).
Christian