Hello @contact-esne,

I appreciate you for getting back and providing updates about the steps you've done on your end. Let me share additional suggestions to get you back on track.

When adding products and services to an invoice, let's make sure to enable the taxable option. This way, it'll take out taxes after clicking the Save option.

Here's how:

1. Open the QuickBooks mobile app.
2. Create an invoice.
3. Add a product or service and make sure to enable the Is Taxable option.
4. Press Add.
5. Choose the sales tax. 
6. Hit Save.

In case the sales tax amount still don't apply, I recommend logging in to your account using the web browser. Please check if the invoice transaction you created shows a sales tax.

If it works fine, please proceed with contacting our QuickBooks Support Team. One of them will be able to take over and assist you further with the mobile app problem.

To reach out to them using the mobile app:

1. Click the hamburger (three lines) menu next to the home icon.
2. Click the question mark icon.
3. Press Help & Support.
4. Hit Contact Us.

I've also attached an article you can use to enter a payment using the QuickBooks mobile app: Receive an invoice payment using the QuickBooks Online app.

Please know that I'm just a post away if you have any other questions. Take care.