Let's consider the system requirements for your mobile device so you can open the app successfully.
But if your device have met the requirements and you're still unable to open the app, I'd recommend contacting our Customer Care Support.
From there, someone can create a ticket ID associated with your account to be submitted to our engineering team for further investigation.
I've got you this article in case you need some ideas about features in the mobile app: Features of the QuickBooks Online app.
Please touch base with me here for all of your QuickBooks needs, @GJC, I'm always happy to help. Thanks for dropping by.