Why do I recieve an error message "Specify all of the new elements to create an array of layers or assign to elements of layers outside of array bounds."

I am new in deep learning and trying to implement a code for transfer learning but whenever i run this code it gives an error message "Specify all of the new elements to create an array of layers or assign to elements of layers outside of array bounds." Error in line 4) , code -

• 1)cnnMatFile = fullfile(tempdir, 'imagenet-caffe-alex.mat');
• 2)convnet = helperImportMatConvNet(cnnMatFile);
• 3)convnet.Layers
• 4)layers(21) = fullyConnectedLayer(5);
• 5)layer(23) = classificationLayer
• 6)allimages = imageDatastore('myimages','IncludeSubfolders', true,'LabelSource', 'foldernames');
• 7)[trainingImages, testImages]=splitEachLabel(allimages, 0.0, 'randomize');
• 8)opts = trainingOptions('sgdm', 'InitialLearnRate', 0.001, 'MaxEpochs', 20, 'MiniBatchSize', 64);
• 9)myNet = trainNetwork(trainingImages, layers, opts);
• 10)predictedLabels = classify(myNet, testImages);
• 11)accuracy = mean(predictedLabels == testImages.Labels)

Please help in solving this.