how can force user to select sub category while posting ad ?

main category and sub-category 

xyz.com/for-sale/laptop

now people just select 1st main category and posting ads so, admin again edit ad & add relevant sub-category.

want user must select main category then sub category but how ...?

Any solution ..?

@Ajit Sahane
As far as I know, until option "enable users to choose parent category" is disabled, user must select subcategory.

Yes done.

want to update error message " Category invalid " 

where is the location of this message?

Update via core translation files