Accessd Query not Working

Experts,

I'm attempting to do a simple correlated update over an Access 2007 table like this:

UPDATE Notes SET Notes.Note_Category_Descripton = DLOOKUP("Note_Categories. Note_Category_Descripton ", "Note_Categories", " Note_Category_ID = " & Notes_Category);


However, I'm told that the field "Note_Category_Description" cannot be updated because the field is "not updateable".  This is the first time I've ever seen anything like this.

Ideas?