This layer is related to another layer by a relationship class and cannot be edited with an ArcGIS Desktop Basic license.
Solution: Upgrade your license to ArcGIS Desktop Standard or ArcGIS Desktop Advanced.
More information: With an ArcGIS Desktop Basic license, you receive this error when you attempt to start editing a feature class that is related to another feature class or table. In addition, it appears if you attempt to edit feature-linked annotation, since a relationship class is used to maintain the link between the origin feature and its annotation.