You'll usually get this error when there's an internet interruption, CyndyR.
Let's try to use other network in accessing QuickBooks. Then, try to sign in again. We can also verify if your computer meets the system requirements for QuickBooks Online app for Mac. After, we can start either to reset your app data to clear and refresh your QuickBooks app cache or uninstall and reinstall the app.
Reset App Data
Click the Help menu.
Select Reset App Data.
If the same thing happens, we can uninstall and reinstall the app. I would also suggest using a browser in signing in to QuickBooks if the issue persists.
Reach out to me if you have other concerns. I'm here to help you!