ConnectBooks couldn't finish because another QuickBooks session had the company file open.
How to fix it
Close any other QuickBooks windows, and ask anyone else using the file to close it.
In QuickBooks, click File, then Switch to Single-user Mode.
Click File, then Update Web Services. Tick your company and click Update Selected.
If a settlement got only part of the way into QuickBooks, delete it and send it again. See How to delete or undo a settlement from QuickBooks.
Frequently asked questions
The error says multiple QuickBooks instances are running.
QuickBooks is open more than once. Close the other instances and run the Web Connector again.