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If you have already added the tax code, we can do some troubleshooting in your mobile app to rule out if it's a cache issue.
To clear the cache, you can follow the steps below.
Power down and restart your device. Then, try to create an invoice and check if you see the sales tax.
Otherwise, uninstall and re-install the QuickBooks mobile app.
You can read this article for more details: Troubleshooting Quick-Start Guide for the QuickBooks Online app
On the other hand, check out this article if you have not added a custom tax rate: Use custom rates to manually calculate taxes on invoices or receipts.
I'll be around if you have more questions about sales tax in QuickBooks Online. Thanks.