With the upcoming April 2021 release, support for Visual Studio 2015 will be dropped. If you’re building your code using a build VM deployed from LCS, you’re using the legacy pipeline. You will have to manually update your build pipeline tasks to use the new version. The steps are fairly simple and outlined in this official docs article. I have a quick video on YouTube to walk you through this as well. There is one little flag that could trip you up, however.