I'm not a 6tin user, but I know that all apps that can access facebook profile info can be found in fb settings.
If you have a computer, this is easier to do there. If not, open the facebook app on your phone and go to settings > privacy settings. It will probably ask you to log in again (that's a good thing). From there, swipe down to see Apps. When you find the 6tin app (I'm assuming it will be the 6tin app, but since 6tin is a tinder client, it may show as the tinder app itself), tap the arrow to the right and verify that nothing is blocked.
You can also try removing the app from fb and setting it up again in 6tin. Last resort... uninstall 6tin and reinstall.
Not the definitive guide to fixing your problem, but general suggestions for shooting trouble.