This happens 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. Make sure there are no other windows open 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