Usually that means there is already a session pending or running on that resource and the run mode of the session you are trying to run conflicts with that of the one running/pending already.
You'll want to look into how run modes work (Exclusive, Foreground, Background). I'll reply again with the name of the document that describes this if I can find it (I think there is a description of it in Webservices but I'll need to go check for a more basic document that describes it).
If there should be no process/session running or pending on the resource already, you may want to show all time in Control Room for that resource to verify there's not a pending session from days ago.
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Dave Morris
3Ci @ Southern Company
Atlanta, GA
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Dave Morris, 3Ci at Southern Company