Before running certain tasks with the PyXLL command line tool you will need to close Excel.
Sometimes Excel does not close down completely and leaves a running EXCEL.EXE process, even though no Excel window is visible.
To kill any remaining Excel processes open the Windows Task Manager by right clicking on the Windows Task Bar and selecting "Task Manager". Under the Details tab look for the EXCEL.EXE process and kill it (you can sort by name by clicking on the Name column). If you do not see the Details tab then click the "More Details" button to reveal it.
Once you have killed any running EXCEL.EXE processes try the command again.