[O] Symbol's function definition is void: org-export-dispatch

[Joseph Vidal-Rosset]

Hello,

For the first time I've tried to export into a .tex file an draft .org. 

But C-c C-e  produces an error message in my emacs :

symbol's function definition is void: org-export-dispatch

I've tried to read the manual, read the discussion about this point via google, and I am lost. 

Can I find some help? I am a beginner... 

Thanks

Jo.