S7 edge & facebook

A

AC Question

Ive recently purchased a galaxy Samsung s7 edge. I've downloaded the fb app. When I sign into it ,it comes up with my old- original Web account. Both Web & fb app have the same email & password.. I cannot access my fb app account. Are there any settings I need to set up on my phone to enable fb app to work... if so ,please give very simple instructions. Don't understand why I can't log in & get my app up & running .. Getting extremely frustrated..