All sales tax agencies, like City, County, and State should show under the Add Agency drop-down. If you can't find the agency you're looking for, let's perform some troubleshooting steps so you can add the agency for sales tax.
There are times that the browser stores frequently-accessed data, thus causing websites, like QuickBooks Online (QBO), to act weirdly. Let's try logging into your QBO account using a private browser.
To use a private browser, here's how:
Google Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + P
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once logged in, go back and try to add the agency for sales tax again to double-check. For more guidance, you can check out this article: Set up and use automated sales tax.