VSTS Build expired
"Build Expired" is a feature of branch policies. Set a build expiration to make sure that updates to your protected branch don't break changes in open pull requests.
Always require a new build
This option sets the build policy status in a pull request to "failed" when the protected branch is updated. You must re-queue a build to refresh the build status. This setting ensures that the changes in pull requests build successfully even as the protected branch changes. This option is best for teams that have important branches with a lower volume of changes. Teams working in busy development branches may find it disruptive to wait for a build to complete every time the protected branch is updated.
Require a new build if older than ... hours
This option expires the current policy status when the protected branch updates if the passing build is older than the threshold entered. This option is a compromise between always requiring a build when the protected branch updates and never requiring one. This choice is excellent for reducing the number of builds when your protected branch has frequent updates.
Don't require a new build
Updates to the protected branch do not change the policy status. This reduces the number of builds for your branch, but can cause problems when closing pull requests that haven't been updated recently.
More details on "Branch Policies"
On the overview of the push request, next to "Build Expired" - click on the ellipses (3 dots to the right) and select "Queue Build". If it fails, pull master, merge it into your branch and re-push.