Data Editor
Missing values for string variables are handled similarily to those for numeric values. Unlike numeric values, blank fields in string variables are not designated as system missing. Rather, they are interpreted as a blank string.

step Click the Variable View tab at the bottom of the Data Editor window.

step Click the Missing Values cell for the sex variable.

step Click the button in this cell to open the Missing Values dialog box.

step Select Discrete Missing Values.

step Type NR for the missing value.

Missing values for string variables are case sensitive. So, a value of nr is not treated as a missing value.

step Click OK to save your changes and return to the Data Editor.

Now you can add a label for the missing value.

step Select the Values cell in the sex row and click the button to open the Value Labels dialog box.

step Type NR for the Value.

step Type No Response for the Value Label.

step Click Add to have this label added to your project.

step Click OK to implement your changes and return to the Data Editor.