Making sure you can assign sales tax to your purchase orders is a great way to make sure your sales tax is correct when filing to the government. I can help go over this with you.
QuickBooks Online makes assigning sales tax to your orders a breeze and helps you save time. As QuickBooks Online is a web based program when issues happen, such as active sales tax codes not showing, it usually is due to your browsers health. In order to fix this you'll need to clear your cache and cookies which can be done by following the steps in this article. You can also try using a different internet browser, such as firefox, to see if it continues to happen. Another step would be to use a private browser such as Google Chromes incognito mode which can be accessed by going to the three dots in the top corner and selecting incognito mode.
Hope this helps!