As a SharePoint Site Owner, you can request the Taxonomy App from SharePoint Store directly from within your site.
Note: If a request has already been approved you can go ahead and add Taxonomy App to your SharePoint site, learn more following this link.
1. Navigate to your SharePoint site and click the gear icon next to the profile icon on the top right corner of the page.
2. In the displayed menu click the Add an app menu item.
3. The My apps page is displayed.
Click the Find more apps in the SharePoint Store link.
4. On the SharePoint apps page, type Taxonomy in the search field and hit the Enter key.
Click the Taxonomy App tile as shown in the screenshot below
5. The Taxonomy App page is displayed.
Click the Request button to initiate the request process.
6. A popup window is displayed to optionally provide more details to the SharePoint Administrator.
After submitting the request it shows up on the Pending requests section of the SharePoint Admin page. Follow this link to learn more about Approving a request to add the Taxonomy App.