Activating LED on signal input=null

Other Hardware LEDs and Multiplexing
5

Thank you all guys, now it's just working fine .
How am I supposed to make it blink under this condition ?

void loop()
{
   
   if((millis() - oldTime) > 1000)
  { 
     detachInterrupt(sensorInterrupt);
        
    flowRate = ((1000.0 / (millis() - oldTime)) * pulseCount) / calibrationFactor;
    oldTime = millis();
    flowMilliLitres = (flowRate / 60) * 1000;
    totalMilliLitres += flowMilliLitres;
    if (flowRate < 0.01) digitalWrite(GREEN,HIGH);   //main condition 
    else digitalWrite (GREEN,LOW);