You tried to launch an Accessory or other program from PC Access. When PC Access tried to launch the program, Windows reported that the program is already running. PC Access tried to find the program's main window but could not.
This is an error MFM does not expect to be very common. Try opening your Windows Task List (in Windows 3.1x, press Ctrl-Esc; in Windows 95 and NT you may press Ctrl-Alt-Del to open the Task List or Task Manager, respectively). See if the program you are trying to run is already visible on the Task List. If it is, try selecting it and see if you can open its main window.
Try closing all programs, exiting Windows, and restarting your computer. Re-open PC Access and try to launch the program again. If the error persists, follow the instructions for an Internal Error.