[O] no beamer export?

[Luca Ferrari]

Hi all,
I'm running emacs 25.1.2 and I've no beamer export in the C-c C-e.
I've tried to include the directives:

#+LaTeX_CLASS: beamer
#+LaTeX_CLASS_OPTIONS: [presentation,smaller]
#+BEAMER_THEME: default

and then do a latex ordinary export (C-c C-e l p) but I got 'Unknow
latex beamer class'.
Am I missing something?

Thanks,
Luca