why showing ??? Undefined function or method 'afb' for input arguments of type 'double'

x = rand(1,512);      % Test signal
J = 4;                 % number of stages
[Faf, Fsf] = FSfarras; % 1st stage anal. & synth. filters
[af, sf] = dualfilt1;
w = dualtree(x, J, Faf, af);    
y = idualtree(w, J, Fsf, sf);   
err = x - y;       
max(abs(err))

[Image Analyst]

afb does not even occur in your code. You forgot to include the error message. All you gave was a small snippet you clipped out but forgot to include the crucial information.