Images of scribes in the ancient world typically depict them as writing on their knee. While this may have been a convention or custom, it is hard to believe that no scribes in the ancient world had access to tables for writing.
Saenger theorizes that a writing table, as well as word spacing, would have been necessary for visual copying using silent reading, rather than copying from dictation as is understood to have been done in the ancient world.
The earliest unambiguous image of a scribe using a writing table appears in the Lindisfarne Gospels, an illuminated gospel book that incorporates word spacing, produced on the tidal island of Lindisfarne off the north-east coast of England, circa 715-720. The image is a portrait of the evangelist Mark. f.93v. For a color reproduction see Brown, The Lindisfarne Gospels. Society, Spirituality and the Scribe (2003) plate 14.
Saenger, Space Between Words. The Origins of Silent Reading (1997) 48.