The main reason for QuickBooks error code 6129 can be inability to connect with the database connection. This commonly happens when the QuickBooks user generates a new and unique company file.

Description of the message: When creating or opening a QuickBooks Desktop Company file and you encounter

Error code 6129, 0: Database connection verification failure

Possible reasons for this error code to occur could be:–

If the installation and download of QuickBooks Pro software is corrupt or incomplete.
If there are changes in the current QuickBooks Pro related software in the windows gets corrupt.
If virus attacks lead to corrupted windows system files.
Due to third party antivirus.
When the network data files are damaged.
If the client’s machines and server are not on the same domain.

Reference Source: https://www.nairaland.com/6328112/solve-quickbooks-error-code-6129