The tooltip explains exactly what the problem is. The output of the FFT function does not match the expected input of the peak detect function. More specifically, the FFT function output is a cluster of values (frequency domain) similar in structure to, but not the same as, a waveform. The peak detect function operates on a (time domain) waveform. You can't wire the two together. You can, however, tie the peak detect to the waveform wire you already have just above that function, but I suspect you weren't looking for a time domain peak.
Here's a way to convert your freq domain info into a waveform datatype. It's kinda ugly but will probably get you started. You may need to experiment a bit to discover exactly how scaling of inputs and outputs (such as width and derivatives) is affected by this attempt to trick the peak detector into operating on freq domain data.
- Kevin P