The type or namespace name 'Azure' does not exist in the namespace 'Microsoft'

If you already have the NuGet package installed, but it's still not recognized, for me it often helps to downgrade the package to a lower version (using the package manager) and then upgrade it back to the original version.

This is especially true if you are cloning existing projects to a new machine.

You have the package for .NET Standard. I believe you need the package for .NET Core for UWP. Microsoft.Azure.DocumentDB.Core v 1.7.1

Please note the package Microsoft.Azure.DocumentDB.Core 1.7.1 is not compatible with the version before Windows 10 Fall Creators Update. So you should set both the Target and the Minimum version to Windows 10 Fall Creator Update Build 16299 for your project.