You probably did not grant access to bitrise.io when you authenticated with GitHub to access the organization (or a repository owned by the organization).
To fix this you should follow this guide: Grant access to a GitHub Organization.
If you have a project with one or more submodules or other private repository dependencies (for example, CocoaPods repositories), you have to grant access to all repositories for a successful...
This guide explains how to grant access for bitrise.io to your Bitbucket team, and to check if you denied access for any reason.
This guide explains how to grant access for bitrise.io to your GitHub Organization. This way Bitrise can access the Organization's repositories.