If you want to use AOT-compiling (AOT = Ahead of Time) for your Xamarin.Android it requires that Android NDK (Native Development Kit) is installed on the machine that will build your app. I using VSTS to build my apps and I using the "Hosted VS2017" build agent. After I enabled AOT and LLVM (Low Level Virtual Machine) to get better performance I got an error that said that Android NDK was missing on the machine.
But after some research I found out that Android NDK was installed on the machine. What I had to do was to point out the path to the NDK. You can do this by creating a build variable with name "AndroidNdkDirectory" and set "C:\Microsoft\AndroidNDK64\android-ndk-r15c" as value.