Quickbooks Sync Tool Error - "A modal dialog box is showing in the Quickbooks Interface"

Cause

When you pressed sync in the sync tool, you also have a critical model dialog open in Quickbooks (such as the vendor portal) at the same time. This is preventing the sync tool from preforming a sync.
 
Note: Some people refer to "model dialog" as "window" or "dialog box".

Resolution

Simply close the dialog box in Quickbooks and try syncing again. If you're unsure which dialog box is causing the problem, save your work and try closing all open dialog's until your left with the "home" screen open and sync again.