I'm not sure why this is posted under the Sage CRM for Sage 100.   In PO Receipt of Goods, if you are receiving a message that says there is a process running and there is not, I suggest reboot the server as the easiest.   There are other ways to clear a locked file, but would require more information and troubleshooting.  I would suggest you contact support if there is more to the situation than just having a process locked.

If that occurs again, you should be able to highlight that task in Master Console and end that "task" if it is truly not running (it should also list the user in Master Console so you can confirm that the task is not minimized or otherwise open on their machine).  If it cannot kill the task, that may be a sign that it is hung up in Windows - it is possible to kill the process id (PID) displayed in Master Console and in Windows Task Manager or a reboot/restart of the server may be needed. 

If this is a repeat issue, you may want to review with Sage 100 support or your Sage partner why this is happening.  Please be sure to gather the exact version of Sage 100 including 32-bit or 64-bit from the Help (drop down menu) - About Sage 100 screen.