This error message typically displays when you already have a QuickBooks company file with the same name open.
Error: COMException (0x8004040A) A QuickBooks company data file is already open and it is different from the one requested.
- Close Quickbooks.
- Open Windows Task Manager.
- Click the Processes tab.
- Find the QBW32.exe process, select the process, and then click End Process.
- Restart QuickBooks and open the file.
For help with using the Task Manager refer to this Microsoft support article.