Thanks for reaching out to us here in the Community. I can help you view invoices on your QuickBooks Online app.
I've checked here, and there are no reports from other users regarding not being able to view invoice using the mobile app. We can perform some basic troubleshooting to isolate the issue.
Let's start with logging out and logging back into your account again. Here's how:
Click the menu option in the upper-left corner.
Select Signout.
Enter login credentials.
Click Sign In.
You can also try reinstalling the app to ensure you have the latest release. For the detailed steps on how to download the app again, I'm adding the article I recommend on this: