First, I'd like to clarify if you're trying to open your QuickBooks Online company? If so, it could be a browser-related issue. We can access your QuickBooks Online account in a private window. This is a special mode that doesn't save the web browsing history on your local device.
Simply press the following short cut keys to open it:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox Microsoft Edge: Ctrl + Shift + P
Safari: Command + Shift + N
If you were able to access your company without getting the same error message, go to your regular browser and clear its cache to prevent any unexpected errors. They can also use other compatible browsers in QBO.
On the other hand, if you've got this error message when trying to open QuickBooks Desktop. Updating QuickBooks Desktop to its latest release will resolve this one. For more details, please consider checking this article: Fix Error 80029c4a. Please note that this article is only applicable in the QuickBooks Desktop US region.
Here are some articles that you can use as references to learn more about handling, transactions, reports, and customers/vendors data:
Just tag my name in the comment section if you have any other concerns or questions. I'll get back to you as soon as I can. Stay safe, info-spacelinktr.