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The course may have been suspended. Please check out this official documentation.
Microsoft has made changes to the services supported by Courseware Marketplace and Skillpipe. We're transitioning our courseware to Microsoft Learn. However, I didn't search for relevant courses in the official link, you could try to research again.
I've checked a lot of courses on internet, they are all paid courses. Please refer: https://www.theknowledgeacademy.com/us/courses/microsoft-sql-server-training/sql-server-2014-performance-tuning-and-optimisation-55144bc-training/
I compared them. The course in this link is officially certified.
Hope this can help you resolve the issue!
Best regards,
Lucy Chen
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