Make sure you arent confusing the SmartView function (built-in to Samsung phone/tablets) with the Smart View app (these are two different confusingly named things).
Some phone/tablet models do not have SmartView functionality built-in.
Sometimes the SmartView icon in Quick Connect gets moved/removed during firmware upgrades etc - check that it isnt on a second page or available with Edit.
If your Samsung device ultimately does not have SmartView, you can download any number of apps from the Play Store and try them out to find one that suits you (e.g. try searching for "samsung mirroring" or "miracast").