Gram is right. You need to complete Highmountain and get Exalted with the race.
If you have not taken any quests with them, you should be able to pick up the starter from the Scouting Map in your class hall, not from an NPC.
Unlocking Highmountain Tauren
The requirements for unlocking the Highmountain Tauren Allied Race are as follows:
Battle for Azeroth: You must have the Battle for Azeroth expansion purchased.
Ain’t No Mountain High Enough Achievement: This requires completion of the Highmountain storyline, which should take a few hours to complete. Reference the Highmountain Overview and Leveling Path Guide if you need assistance with any quest.
Exalted Reputation with Highmountain Tribe: Highmountain Tribe reputation can be attained mostly while completing the campaign step, while the rest of the reputation needs to be obtained via Highmountain World and Emissary quests, and using reputation items like
Highmountain Tribe Insignia. Reference the Highmountain Tribe Reputation Guide for details on how to reach Exalted as quickly as possible.
Allied Races: Highmountain Tauren: You must then complete a special recruitment quest which demonstrates why the Highmountain Tauren have decided to join the Horde and expands a little more on the story from Spiritwalker Ebonhorn.