Why this multithresh function doesn´t work for my image?

Dear all,

I have this image

and I would like to use multithresh function for this. But this error appeared:

Undefined function 'multithresh' for input arguments of type 'uint8'.

Error in Otsu_databaze (line 22)
level = multithresh(I);

This is my code:

clc;close all;clear all;

I = imread('CT_sken_zkusebni2.jpg');
imshow(I)
I = I(:, :, 2); 
level = multithresh(I);
seg_I = imquantize(I,level);
figure
imshow(seg_I,[])

I also attach my image.

Thank you for your answers.