Why is my laptop fan runs fast continuously

This is happening because your laptop is under a big load or it can’t breathe. Make sure nothing is blocking the fan vents and check for viruses/malware. If nothing above worked then the vent inside the laptop is probably dirty not allowing proper airflow. You can find guides on iFixit and YouTube on how to open up the laptop. If not comfortable on opening up the laptop to clean it, you can take it to a local repair shop. Hope this helps.