This layer has the same data source as a layer inside a basemap layer. You cannot start editing on this workspace while this layer is in a basemap layer.
Solution: You need to either move the layer out of the basemap layer or remove the duplicate layer (the one not inside the basemap).
More information: You encounter this error when you have multiple layers that reference the same underlying feature class and one of the layers is inside a basemap layer. For example, layer 1 and layer 2 both point to the same feature class, but layer 2 is inside a basemap layer. There are two ways to resolve this issue, depending on whether you need to edit this feature class. To be able to edit it, you must move layer 2 out of the basemap layer. However, if you do not need to edit layer 2 and want to keep it in the basemap layer, remove layer 1 from the data frame.Feedback on this topic?