If you are using a USB microphone put the input volume control to its unity gain (0 dB) setting, which is selected by default when the mic is first plugged in. The input level with a USB mic will usually be around -30 to -50 dBFS. If not using a USB microphone the input level should ideally be between -30 and -12 dBFS when the cal signal is playing. If it is lower or higher adjust the Input Volume either using the REW control (if enabled) or your interface or the OS audio level controls. Levels higher than -12 dB may result in input clipping during measurement, levels lower than -30 dB give noisier results, but neither is a hard limit.
Press Next to start the calibration signal or Cancel to quit.