Already reported here: https://community.developer.atlassian.com/t/the-repository-https-packages-atlassian-com-atlassian-sdk-deb-stable-release-is-not-signed/
Workround:
Try to use https://packages.atlassian.com/atlassian-sdk-deb instead of https://packages.atlassian.com/debian/atlassian-sdk-deb