ESP8266 can't detect DS18B20 Temperature Sensor

Other Hardware Sensors
20

on an ESP8266 GPIO 4 is D2, so actually D2 = 4.
To confirm, plug in the nodeMCU and see the result of

Serial.prinln(D2, DEC);

So true. (actually none of the GPIO pins on that side of the board are usable)

should be

OneWire oneWire(D6); // pin D6

or

OneWire oneWire(12); // pin D6