Why there is no channel 37 in WLAN 5 GHz band. what is the reason behind it
The original channel designations assumed 5 MHz spacing, and were made long before anyone thought of Wi-Fi. Since Wi-fi signals are 20 MHz wide, they take up 4 channels.
So there is a channel 37 (and 38, and 39 ...), but it is part of the 20MHz bandwidth of a single Wi-fi signal (using channel 36).