This layer cannot be edited because it 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: A layer in a basemap layer cannot be edited. If you have other editable layers in the map, you can edit them, but you are unable to edit the layer in the basemap layer.
In some cases, this warning is shown in conjunction with an error that prevents you from starting the edit session altogether. For example, if the layer in the basemap is also in a geometric network or topology, you cannot start editing at all. You must move that layer out of the basemap layer before you can start editing.