f(x,y):=x.y is refused "unauthorized change : the variable type is invalid" (but seems working on many examples I see over the Internet...)

Question

I erased all the variables of my scratchpad, with ClearAZ,
and attempted to enter the multivariable function f(x,y) := x.y that I see everywhere over the Internet on my TI-nspire CX II-T.

But, myself, I'm receiving the message: "unauthorized change : the variable type is invalid"
(I'm translating the message from French, I've received it verbatim like: "Changement non autorisé : le type de variable est invalide")

Why? And what should I do?
I have no clues.

Answer 1

Epilogue:
I've discovered that I was able to define g(x,y) := x.y, but ClearAZ or DelVar weren't able to really delete my f function, that was, in fact, still linked to a graphical representation.

I had to go on the graph tab, then ensure to do the action "Erase all" to make the function really disappear from nspire memory.

And then defining f(x,y) := x.y became possible.