You can't find the peak of a single data point. It is telling you to provide more data points in order for the function to work. You can use the buffer block in simulink to build-up some data points. I suggest 1 period of the oscillation, so it can capture both the max and min of the signal. Note however, that this adds a latency to your real-time model (not that it isn't still real-time in terms of throughput). I'm not familiar enough with Simulink to provide another solution.
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