Check your Microsoft.TeamFoundation.Framework.Server.dll version in the Application Tier\Web Services\bin folder to confirm your current patch level.
While these patches keep your server secure, TFS 2015 is now several generations behind. Microsoft now brands the platform as Azure DevOps Server, with the 2022 and 2024 versions offering massive improvements in pipelines, YAML support, and Git integration. TFS-Update_4a-pc.zip
Before you unzip TFS-Update_4a-pc.zip and run the installer, ensure you have completed these steps: Check your Microsoft