A layer in a basemap layer is joined to an editable table or layer in the map.
You cannot start editing on this workspace while this layer is in a basemap layer.
Solution: Move the layer out of the basemap layer. You can drag layers in the table of contents to move them in and out of basemap layers.
More information: Another way to solve this is to remove the layer from the basemap layer, remove the join, then move the layer back into the basemap layer. You cannot edit the layer in a basemap layer, but this method allows you to edit the other layers in that workspace.