May be some 3rd party app will be able to do it.. If i am not wrong J6 must be running on Oreo.. and Night Mode was released with One UI 1.5 on Android Pie.. So officially Night Mode wont be coming to this phone..
If you want in your current phone then there is only one way just install 3rd party rom. It will give you better features with latest android. You can search it for your phone on web and than istall it through computer. You will find installation guide with rom. Some rom are far stable than official samsung rom..