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.
When setting up the QuickBooks Desktop integration, you might encounter the following error after attempting to open the downloaded config file: QBWC1039: There was a problem adding the application. Check QWCLog.txt for details. Error connecting to QuickBooks. Error message received from QuickBooks was: This application has not accessed this QuickBooks company data file before. The QuickBooks administrator must grant an application permission to access a QuickBooks company data file for the first time. This error occurs because you did not correctly sign in to QuickBooks Desktop using the built-in “Admin” user before starting the setup process. Even if your current user has administrative privileges, the username must specifically be “Admin” - this is a QuickBooks Desktop requirement that cannot be bypassed.

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Solution: Sign in as the “Admin” user

1

Sign out of your current QuickBooks Desktop user account

2

Sign in using the QuickBooks built-in "Admin" user account

Sign in using the QuickBooks built-in “Admin” user. The username must be exactly “Admin” - not any other user, even if it has administrative privileges.
Signing in as the “Admin” user is only needed for this one-time setup process. After setup is complete, you can return to using your regular QuickBooks Desktop user account.
QuickBooks Desktop sign in as Admin
3

Re-open the QuickBooks config file

Re-open the QuickBooks config file again from your Downloads folder (it has the file extension .qwc). This is the file you just downloaded from the QuickBooks Desktop setup guide. Now, QuickBooks Desktop should no longer show the error shown above and you can continue setting up the connection.
4

Continue where you left off in the QuickBooks Desktop setup guide

After you complete the setup process, you can return to using your regular QuickBooks Desktop user account.
Following these steps will ensure you’re properly signed in as the QuickBooks “Admin” user, which is required to grant the Web Connector permission to access your QuickBooks company data file for the first time.