Creating a loop in the Portsim

I'm working on a retirement portfolio problem. The code below essentially calculates 100 simulations of 15 years of portfolio returns for 126 different portfolio combinations. The output matrix is 15x126x100 in dimensions. What I am trying to do is to add a loop that calculates the results for different portfolio durations. I have to test the 126 portfolio combinations for 15,20,25,30,35 years but I am unable to loop that in. I keep getting an error regarding RetIntervals input in the Portsim code. Can someone help me with this?

NumObs = 15; %Duration of the portfolio

NumSim = 100; %Number of simulations

RetIntervals = 1;

NumAssets = 5; %Number of assets

rng('default');

RetExact = portsim(ExpRet, ExpCov, NumObs, ... RetIntervals, NumSim, 'Exact');

M = assetallocations(5,5); %Returns 126 combinations if weights of 5 assets

m = size(M,1);

for i=1:NumSim

    for j = 1:m
    PortRetComb(:,j,i) = RetExact(:,:,i)* M(j,:)'; %multiplies the simulated returns to each combination to get portfolio returns%
    end
end