This issue can be generally come up because of a configuration issue. To find out exactly why this issue is coming up, enable and check the log files.
Steps to get Jira OAuth Plugin logs are given here: https://faq.miniorange.com/knowledgebase/get-jira-oauth-client-add-logs/
Some of the possible reasons are:
1. Mapped Domain is not allowed – Check Allowed Domains in Sign In Settings tab. The email address domain which you are attempting to SSO with may not be included in mapped domains.
2. OAuth Provider is not configured correctly – Verify OAuth configurations in the Configure OAuth tab
3. License is not installed – The add-on does not have a valid license installed. An evaluation license is necessary to perform SSO using the add-on. You can get a license from My Atlassian <add a link here> Licenses.
4. User creation is not allowed – The user you are trying to login with does not exist and disable user creation option is checked. You can uncheck Disable User Creation checkbox in User Groups tab. You can also verify your user’s username and email mapping to make sure the correct value is being used to verify if the user exists.
If you are still not able to resolve the issue, please contact us on firstname.lastname@example.org