If your UWP application built with App Center Build service started failing to install on a Windows machine and it is identified as untrusted around Mar 11, 2021, this issue might be because of a new update on Visual Studio 16.9.x (msbuild v16.9.x).
While Visual Studio team is working on the hotfix, here's what you can do to work around the issue.
Workaround
1. Create a file called Directory.Build.targets in the same directory as your .sln.
2. Paste the following text inside.
<Project>
<Target Name="_ValidateSigningCertificate" />
<Target Name="_RemoveDisposableSigningCertificate" />
</Project>
3. Save and commit the file.
4. Trigger a new build if your build does not automatically start when a new code is pushed.
Hope this article was helpful. If you're still facing the issue, you can also read more information about how UWP application is signed in general or open a support ticket in the App Center portal under the help menu. We'd be more than happy to assist you on your problem.