Problems with controlling LED with soil moisture sensor
A good way to start debugging this sort of problem is to print out debug statements that say what the hardware should be doing. If the debug statements print out as expected then it's probably hardware, if not, then it's probably your code, e.g.

if (percent &lt; threshold){
  Serial.prinln("LED on.");
  digitalWrite(LED_BUILTIN,HIGH);
}
else {
  Serial.prinln("LED off.");
  digitalWrite(LED_BUILTIN,LOW);
}