I would like to know if you encounter an error. If so, we have a reported issue about an error when adding tax rate and agencies in QuickBooks Online (QBO). Our engineers are still working for a fix.
Please give us a quick call so you'll be added to our list of affected users. Go to Help then click Contact us.
On the other hand, if you did not encounter any error, we can try basic browser troubleshooting for this. Log in to QBO via private browser. If you are able add a tax, you can clear the browser's cache. Alternatively, you can use other browser.