1 Your under 18
2 It is only available on android and IOS, Not on windows
3 Your Facebook app is not updated, return to main menu and
click   to update 
4 Your state is not single
5 Enable Facebook App Notifications in the local configuration of your phone. If App notifications has been turned off or disabled, make sure that Facebook is exempted from that list, or you can try to Enable again Facebook App notifications to be able to use Facebook Dating service.
6 Clear Cache on your Phone, for apps to run faster. your mobile phone stores data from the apps. However there are instances wherein the data stored gets corrupted causing the app to stop functioning properly. Clearing cache on your mobile phone can sometimes resolves this. Try to check if dating feature will already show on your device and working properly.
Uninstall and Reinstall Facebook App, sometimes uninstalling and reinstalling an apps on your Android and iOS device resolves any conflict that arises with the app. That way you’ll get a new and updated version as well to be able to use the dating feature. Simply tap the uninstall tab on your mobile.
8 Create a new user wit other name