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First, if you're using the QuickBooks Online (QBO) Desktop App, please note that we already discontinued it. This means there'll be no more software updates nor customer support. Although you can still use it, you're more likely to encounter errors. Visit this page for more details: QuickBooks Online Mac and Windows Desktop App update.
If you're using the mobile app, make sure your phone's operating system is updated. Also, go to the App Store or Play Store to update the QBO mobile app, too. If the same thing happens, you'll want to uninstall and reinstall it.