There are pending geoprocessing operations running in the background.
Solution: You must allow all geoprocessing tools in the background to complete (or cancel them) before beginning an edit session.
More information: When background processing is enabled, geoprocessing tools execute in the background, and you can continue working with ArcMap (or other ArcGIS applications, such as ArcGlobe) while a tool executes. When a tool is executing in foreground mode, you must wait until the tools stop executing before you can continue with other work.
When you are in an edit session, geoprocessing tools run in the foreground.