Contact a user that has TDAdmin access for the project. They can log in and go to Tools > Customize > Requirement Types and review the list of defined Requirement types. If "Functional" doesn't appear there, that is why you can't see it. It could be added by a TDAdmin.
If the Requirements List in Customize does include "Funtional", then some other customization has been implemented that prevents you from seeing it. A user with TDAdmin rights would be able to investigate that also.