To add a Term Store Administrator, follow these steps. By default, Office 365 and SharePoint Online do not have any users in this role. Before you can administer any Term Store feature, you'll need to add an account that you can use to the Term Store Administrator group. Follow these steps to add Term Store Administrators:

1. Open the Term Store management tool by navigating to the SharePoint admin center, selecting Classic features, and then selecting Open under Term store:

2. Select the taxonomy and then, under Term Store Administrators, add users by adding usernames or addresses and separating them with semicolons:

3. Click Save.