Hi there, scgearheadz-gmai.
Good to see you here! Are you trying to receive and apply payment against an invoice using the mobile app? Let me show you how to do it.
For Android phones, click the Plus (+) icon and select Invoice payment. Then, you can select Charge payment if you have a QuickBooks Payments account or Record payment to manually track it.
If you're using iOS, from the Menu, select Invoice Payments. You can also check these links for more details:
Also, these links will show you the steps you need to do if encountered issues with either the mobile or desktop app.
Keep on posting here if you have more concerns. Wishing you all the best!