The steps provided by my colleague @AlcaeusF above can help migrate the sales tax feature of those existing QuickBooks Online account to the old one. For those newly created QBO account, like yours, the Automated Sales tax is a built-in feature.
Thus, the manner of switching it back on your own is unavailable. However, you can use the override capability available on the transactions to ignore the automated calculation.
For more information, check out the last part of the Frequently Asked Questions section of the article below:
If this won't work for you, I'd suggest contacting us. One of our support specialists can securely access your account and do the process on your behalf.
You can let them know about the steps you've taken and you're currently using a new account. You'll also want to give the link of this post (look at the address bar above), so you don't need to re-explain the situation.
Our most-up-to-date contact details can be found here:
Welcome aboard to the QuickBooks Community. Thanks for getting back to us about the Automated Sales Tax feature.
The Automated Sales Tax feature is released to existing users by batch. If you want to migrate to the new one, you can let our Phone Support Team know about this. They will pull up your account and ask for some information to notify our Sales Tax Team about your request.
Automated sales tax currently supports customers on Accrual basis method. If you're on the Cash basis, enabling AST is unavailable. However, our QuickBooks Support Team will be able to add you to the priority list for migration once it's available.
You can reach them by visiting the article I've attached below: