@AusPhil Thank you for reaching out to us, As per the issue details mentioned above, looks like Install the extension step is not completed, due to which the below option is greyed out.
Refer to this article for more steps - https://learn.microsoft.com/en-us/azure/active-directory/saas-apps/atlassian-cloud-tutorial related to Azure Active Directory SSO integration with Atlassian Cloud
Let me know if you have any further questions, feel free to post back.