\uD83E\uDD14 Problem
You are unable to add a new GitHub organization to your Confluence with Github Macros for Confluence.
\uD83C\uDF31 Solution
This issue happens mostly due to an active session of Github in your browser. By default, Github Macros for Confluence takes the current session to access any organization/data.
Log out of any active session of Github in your browser.
Follow the steps mentioned Configuring Github Macros for Confluence (Cloud)
You will need administrator access to configure this