Hello, Allin;
 
If you open the
gretl console and do

pergm varname

do you get the error?  Or is it only when you select the
periodogram option from the menus?  
 
Yes, I get the same error when I do in the console or with the menu
 
Are you running gretl in Spanish or in English?
I'm running Gretl in Spanish
 
Do you also get an error if you call for the correlogram of the
variable, from the menu?

No, if I call for the periodogram I don't get any error
 
 
Cheers
Javi