I can think of two things you can check:
1) It may not be string variable, but a numerical variable in disguise; what you saw on the data page may actually just be value labels being displayed. Could you go to the "Variable" tag (located at bottom left) and reconfirm what type of data that is?
2) If you are working on a data hosted remotely there might be a chance that you can't overwrite the data due to it being write-protected. Try use "File > Save As" to safe a file to your own computer, and then see if you edit the information.