This guide is provided by Conductor, the service that powers the QuickBooks Desktop integration for many business tools, including the one that sent you here.
If you were directed to this page, it was likely because you encountered the following error attempting to sync with your QuickBooks Desktop: 
We could not connect to your QuickBooks Desktop because either QuickBooks
Desktop is closed and we could not start it, or an error popped up on your
Windows computer.
Alternatively, you might have been directed to this page for the following error (which is the exact same error; the message above is merely the user-friendly version): 
(0x80040408): Could not start QuickBooks.
Follow these troubleshooting steps to resolve the issue:

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Troubleshooting steps

1

Ensure only one version of QuickBooks Desktop is installed

For the integration to work, only one version of QuickBooks Desktop can be installed on your Windows computer. Having multiple versions installed can cause the error above and several other issues. Please complete this guide to ensure only one version of QuickBooks Desktop is installed on your computer. If the Web Connector ever crashed when syncing, this is almost certainly the culprit.After completing the guide, try syncing with QuickBooks Desktop again.
2

Recreate the Web Connector log file

If you continue to see the same error when syncing, you may need to recreate the Web Connector log file. Please complete this guide to delete the log file. If you recognize the error pop-up below, then this is certainly the culprit. Even if not, it’s a good idea to delete the log file (as described in the guide) to ensure it’s not corrupted.There was a problem with the log file. QuickBooks Web Connector will continue without a log fileAfter deleting the log file as described in the guide, try syncing with QuickBooks Desktop again.
3

Ensure QuickBooks Desktop and the Web Connector are not set to ‘Run as administrator’

For the integration to work, QuickBooks Desktop and the QuickBooks Web Connector must launch with the exact same Windows permission level. If one is set to “Run as administrator” while the other is not, you will see this “Could not start QuickBooks” error. Please complete this short guide to ensure “Run as administrator” is disabled for both programs.After completing the guide, try syncing with QuickBooks Desktop again.
4

Do you see a popup on your Windows computer saying 'Do you want to allow this app to make changes to your device?'?

If you still see the same error when syncing, check if the dialog box shown below appears on your Windows computer. If so, please complete this guide to resolve that issue. Then, try syncing with QuickBooks Desktop again.QuickBooks Desktop - Do you want to allow this app to make changes to your device
If you continue to see the error above after trying all of the solutions above, please email us at support@conductor.is.