If you have a well defined build process, and yet your build is failing in the Initalize Job step with error messages like as shown below.
Issues like this are really puzzling as builds that worked in the past are now no longer working and it seems like nothing has changed!?
2017-11-15T11:31:09.4021313Z ##[section]Starting: Initialize Job
2017-11-15T11:31:09.4177514Z ##[debug]Primary repository: Client.Product.FabricApp. repository type: TfsGit
2017-11-15T11:31:09.4177514Z Prepare build directory.
2017-11-15T11:31:09.4490007Z ##[error]Object reference not set to an instance of an object.
2017-11-15T11:31:09.4490007Z ##[debug]System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.VisualStudio.Services.Agent.Worker.Build.TrackingManager.Create(IExecutionContext executionContext, ServiceEndpoint endpoint, String hashKey, String file, Boolean overrideBuildDirectory)
at Microsoft.VisualStudio.Services.Agent.Worker.Build.BuildDirectoryManager.PrepareDirectory(IExecutionContext executionContext, ServiceEndpoint endpoint, ISourceProvider sourceProvider)
at Microsoft.VisualStudio.Services.Agent.Worker.Build.BuildJobExtension.InitializeJobExtension(IExecutionContext executionContext)
at Microsoft.VisualStudio.Services.Agent.Worker.JobExtension.<InitializeJob>d__9.MoveNext()
2017-11-15T11:31:09.4490007Z ##[section]Finishing: Initialize Job
Solution
This means your working folder on the Build Agent is corrupted. The best option is to delete the work folder (this normally is C:\agent\_work) and trigger a new build.