Build issue in VSTS build agent on Mac (Xamarin)

Introduction

Basically, I'm not able to setup continuous integration for a Xamarin.iOS project using VSTS and xbuild on macOS.

Current Setup

I've set up a VSTS build definition to build a very simple project (template actually) on a Mac Mini with the latest Apple macOS 10. Xamarin is installed and I was able to build and deploy a sample project onto my iPad. The same applies for XCode as well. Below is my whole continuous integration chain:

1. Upload code to Bitbucket
2. VSTS build definition gets notified about new change
3. VSTS build definition connects to the build agent on macOS
4. build agent builds the project

Problem

The build always fails with the following message:

[error]XamariniOS task failed with error Error: Failed which: Not found xbuild: null.

Unfortunately, I'm not able to find out what exactly failed. The source code is under ~/myagent/_work/1/s. The solution file to be built is also there and I'm certain that this is found.

The build agent on macOS just reports:

Job build completed with result: Failed

Actual Question

What is missing in my setup? Where can I get more info to track down that issue?

The error message actually says that xbuild was not found on the build system. The solution is to provide the path to xbuild in the build task settings. There is an extra input field for that in the Advanced section.