Let me welcome to the Online Community, bart2.
Performing these basic troubleshooting steps can resolve unexpected behavior when using the mobile app. Here's how accomplish this task.
Next, go back to the Expenses menu to add the transaction and then pay it using a credit card. If the issue persists, I suggest uninstalling and reinstalling the app to start on a clean slate.
To download the application, follow the process in this link: QuickBooks Online mobile app.
For future reference, I’m adding an article with links to different processes you can perform using the mobile app: Mobile applications hub.
Alternatively, use the browser to open your company. From there, add the entry and pay it using the desired payment method.
Reach out to me again if you have additional questions or concerns. Please know I’m ready to help you. Wishing you the best.