Check to see if you have another function (or something else) with the same name.
To do that, run this:
which nlarx -all
from your Command Window or a script.
The correct result should be something like this:
which nlarx -all
If it returns anything else, that will point to the problem.
Meanwhile, that example works here —
load twotankdata;
Ts = 0.2;
z = iddata(y,u,Ts);
sysNL = nlarx(z,[4 4 1])
sysL = arx(z,[4 4 1]);
figure
compare(z,sysNL,sysL)
.
