The NA (Not Applicable) box is specific to slider questions (or matrix with sliders).
Given the slider being positioned by default on a value, the NA box indicates the difference between a valid answer and a respondent who does not know what answer.
Note that the NA box is not a collected answer. It simply means that your respondent does not know or is unwilling to answer.
Making your slider question mandatory will not display the NA box as your respondents will have to move the slider to answer.