The most common cause of this is that the wrong efiling credentials are being used.

If you open the accounting period and go to efile / transmit Companies House XBRL it will open the window with the presenter ID and presenter authentication code.

These details are what have been provided to you when you registered with Companies House for a third party efiling account. The presenter ID will be 11 digits and the presenter authentication code is 11 characters. Any letters need to be entered in upper case.