If your MLS requires an ABM (Answer Back Message), your copy of PC Access should have come with an ABM file (PCAWIN.ABM). The ABM file contains an encoded copy of your PC Access serial number. Thus a particular ABM file works with one and only one copy of PC Access.
This message appears if the serial number in the ABM file does not match your PC Access serial number. This will happen if you copy your ABM file from your computer to another computer with a different copy of PC Access. If you are sure the above reason does not apply to you, follow the instructions for an Internal Error.