error using "fitlmematrix" for fitting linear mixed-effects model

I am trying to run "fitlmematrix" for a simple case as shown below, but I am getting this error: ??? Undefined function or method 'fitlmematrix' for input arguments of type 'double'. my code:

X1 = [1 2 3 4 5 6]; X = [ones(6,1) X1]; Y = [5 8 2 9 4 5]; Z = [1 0 0 1 0 0 0 1 0 0 1 0 0 0 1 0 0 1];

lme = fitlmematrix(X,Y,Z,[]);

I don't understand the problem :-( Any help is appreciated!