If you met the system requirements, try clearing the cache in your QuickBooks app to refresh the connection.
Choose Reset App Data.
If the same thing happens, you can uninstall the app again. Then, manually update it by downloading the new Mac App for QuickBooks Online (QBO). You can check out this article for the steps and details: Install QBO for Mac app.
If the problem persists, I'd suggest reaching out to our Technical Support Team. They'll further investigate what's causing this issue.
Here's how to reach them:
Click the Help menu in the upper-right hand corner of QBO.
Click Contact Us.
Enter a brief description of your concern and click Continue.
Select the option to contact us or receive a callback.
In the meantime, you can use the web browser to access your QBO account.
Feel free to visit the Community if you need anything else.