Mobile targeting isn't available natively in Target Standard yet. Instead, create a reusable segment in Target Classic (using the links Rahul shared) and then that segment will appear as an Audience in Target Standard automatically. You can then target a test using that audience.
Stay tuned for updates on mobile targeting moving into Target Standard--we're working on it!
Thanks for reaching out to the Adobe community.
Please have a look at the content on the following links: