This might happen if:

- QuickBooks isn’t running.

- No company file is open.

- An open window is preventing the connection.

What you can do? 

1. Exit and reopen QuickBooks.

2. Sign in to your company file.

3. Close all windows in QuickBooks.

Note: If you still get this error, reach out to Intuit Support.

Reference: https://quickbooks.intuit.com/learn-support/en-us/set-up-payments/fix-common-web-connector-errors/00/203305