Welcome to the ODK forum, @Mosab_Daoud ! We're glad you're here. When you get a chance, please introduce yourself on this forum thread. I'd also encourage you to add a picture as your avatar because it helps build community!
In a multi select question, you can specify a combination of answers that is not allowed- if option "none" is selected, the respondent will not be permitted to select any other answer options . In contrast, if "none" is not selected as one of the answer options, the respondent can select any other answer choices. You can add the constraint for additional options. You should adjust the values (here "none" and "dk") to match your survey.
if(selected(., 'none'), count-selected(.) = 1, count-selected(.))
and (if(selected(., 'dk'), count-selected(.) = 1, count-selected(.)))
constrain_if_dont_know.xlsx (9.9 KB)