I'm trying to use Microsoft Azure with HoloLens.

Tutorial: Detailed Steps to Create a New HoloLens Unity App with Azure Spatial Anchors
I was trying to share objects between devices with reference to

using Microsoft.Azure.SpatialAnchors;
The type or namespace name 'Azure' does not exist in the namespace 'Microsoft'

And an error occurred.
I don't think it's a typo. . .
What can be considered as the cause?