Single User Mode:

Ensure that all users are logged out of QuickBooks.

Restart your computer to make sure that any lingering processes are closed.

Open QuickBooks in Single User Mode and try to open the company file.

Use QuickBooks File Doctor:

QuickBooks File Doctor is a tool provided by Intuit to automatically diagnose and fix common QuickBooks file errors.

Download and run the QuickBooks File Doctor tool. It will guide you through the process of scanning and repairing your company file.

Update QuickBooks:

Make sure that you are using the latest version of QuickBooks. Updates often include fixes for known issues.

Go to the Help menu and choose "Update QuickBooks Desktop." Follow the on-screen instructions to download and install the updates.

Check for Network Issues:

If you are using QuickBooks in a multi-user mode over a network, check for network issues that might be causing the error.

Ensure that all computers are connected to the same network and that they can communicate with each other.

Regards,
Peter