Error 1635: This update package could not be opened...

This error occurs when installing TurboTax. The full message reads:

Error 1635: This update package could not be opened. Verify that this update package exists and that you can access it or contact the application vendor to verify that this is a valid Windows installer update package.

This problem occurs when a prior TurboTax installation did not finish installing or uninstalling, leaving it in a broken state. The residual files on your computer are detected by the TurboTax installer, resulting in the message.