Problem with idnlgrey function error

Good day, I'm a Matlab beginner

I'm trying to use matlab to estimate my experimental data with some model with the function named idnlgrey with this example : https://kr.mathworks.com/help/ident/ug/represent-nonlinear-dynamics-using-matlab-file-for-grey-box-estimation.html

I made the model named Threelevelmodel.m

function [dx, y] = Threelevelmodel(t,x,u,k1,k3,k13,k31,varargin)

y = [x(1); x(2)];

dx = [(-k3-k31)*x(1)+k13*x(2); k31*x(1)+(-k1-k13)*x(2)];

And I saved this file,

Then the main code

FileName = 'Threelevelmodel';
Order = [2 0 2];                            % [ny nu nx]
Parameters = [0.011; 0.011; 0.09; 0.147];   % Initial parameters
InitialStates = [5640000000; 42500000];     % Initial states
Ts = 0;                                     % Time-continuous system

nlgr = idnlgrey(FileName, Order, Parameters, InitialStates, Ts);

When I tried this code, I got the error like

함수 'idnlgrey'은(는) 'char'형 입력 인수에 대해 정의되지 않았습니다.

It says that the function 'idnlgrey' is not defined for the input argument 'char'

However, no matter how hard I read the matlab description of idnlgrey, the 'Filename' must contain a char variable

https://kr.mathworks.com/help/ident/ref/idnlgrey.html?searchHighlight=idnlgrey&s_tid=srchtitle#burmdeb-1-FileName

How can I solve it? Thank you

[Rik]

This time I edited your question for you. Next time, please use the tools explained on this page to make your question more readable.

In general that error message means that you don't have the function. Did you make sure you have installed the System Identification Toolbox?