If the error message is linked to file-extension of Company file, then consider changing the File extension and saving the Company file with a correct extension
Verify that the company File is opening on another system
Re-install QuickBooks if the File is not up-to-date
Save ‘QuickBooks for Macintosh’ in Mac-system and not Windows-based system and vice versa
Use a proper antivirus update before opening the Company file
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