MATLAB append for ANN

I am not sure where to ask MATLAB questions, so I am posting here.

I am training an ANN, and I want to have different instances of training. In each instance, I want to find the maximum difference between the actual and predicted output. Then I want to take the average of all these maximums.

My code so far is:

    maximum = []; 
    k=1;

    for k = 1:5
      %Train network
      layers = [ ...
        imageInputLayer([250 1 1])
        reluLayer
        fullyConnectedLayer(100)
        fullyConnectedLayer(100)
        fullyConnectedLayer(1)
        regressionLayer];

      options = trainingOptions('sgdm','InitialLearnRate',0.1, ...
        'MaxEpochs',1000);
      net = trainNetwork(nnntrain,nnnfluidtrain,layers,options);
      net.Layers

      %Test network
      predictedn = predict(net,nnntest);
      maximum = append(maximum, max(abs(predictedn-nnnfluidtest)));

      k=k+1
    end

My intent is to produce a list named 'maximum' with five elements (the max of each ANN training instance) that I would then like to take the average of.

However, it keeps giving me the error:

wrong number of input arguments for obsolete matrix-based syntax

when it tries to append. The first input is a list while the second is a 1x1 single.