Thanks for taking the time to reach out to the QuickBooks Community for support.
To clarify, are you able to log into QuickBooks Online using a web browser? If so, let's try some basic troubleshooting steps for your mobile app to clear the error.
Here are some quick steps to fix most app-related issues:
Change the Wifi data connection on your device to 4G (varies by device).
Close and restart the app.
Clear app data. To do this:
Open your device Settings.
Click on Apps (varies by device).
Choose QuickBooks app from the list, then choose Storage.