https://www.arduino.cc/en/Tutorial/DigitalPotControl

The code needs a delay to work:

void digitalPotWrite(int address, int value) {
// take the SS pin low to select the chip:
digitalWrite(slaveSelectPin, LOW);
delay(100);
// send in the address and value via SPI:
SPI.transfer(address);
SPI.transfer(value);
delay(100);
// take the SS pin high to de-select the chip:
digitalWrite(slaveSelectPin, HIGH);
}