By default, ArcMap edit sessions are set to perform versioned edits. This means you can only edit data that has been registered as versioned.
To perform nonversioned edits on nonversioned data, you must first set an option on the Versioning tab of the Editing Options dialog box. Essentially, you must turn off versioned editing by unchecking Edit a version of the database with the ability to undo and redo. If you later need to perform versioned edits, you'll need to turn this option back on.