I may be wrong, but I thinkrandint was part of the Communication System Toolbox and its functionality has been replaced by therandi function, for which you can find detailshere .
According to the Communications Toolbox Release Notes the randint function was removed in release R2016b. The suggested replacement is randi. Note that the order of the inputs to randi and the order of inputs to randint are different. The range is the third input to randint but the first input to randi (and it behaves a little differently if that range input is negative), so converting your code is not as simple as doing a Find and Replace.
Here's the documentation for randint in release R2012a for reference.
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