If you are unable to sync with Bill.com it may be necessary to reauthorize the program to communicate with your QuickBooks company file. Before this is done you may receive a message when attempting to connect to your company file that "The QuickBooks file is inaccessible".
First, open QuickBooks and log in as Admin and enter Single-user mode. This action can only be performed by the admin user in the file.
Once you are logged into your company file, go to the Edit menu and select Preferences:
On the preferences screen, select Integrated Applications on the left, select the Company Preferences tab, then find "Bill.com", click on it, and then click Remove:
Next, launch the Bill Dashboard software. Click next through the prompts until Quickbooks pops up with the application certificate window. Select the option to Yes, always allow access. Once you click Continue and Done, you will be able to sync with Bill.com again: