As the error clearly states, it seems that your data contains a mixture of grayscale and RGB images, use ColorPreprocessing Name-Value pair argument to ensure that all output images have the number of channels required by imageInputLayer of alexnet.
Refer to the description of ColorPreprocessing of AugmentedImageDatastore and Import Data into Deep Network Designer for more information.
net = alexnet;
inputSize = net.Layers(1).InputSize
inputSize = 1×3
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