I made a door from a Cube. How do I make it solid?

The door's collider must not be a trigger, or the player will pass through it. If you need a trigger to detect when the player is inside the range, add a trigger volume to the door without replacing the original door collider: select the door object and create a new collider (menu Component/Physics/Box Collider or Sphere Collider - you can't have two colliders of the same type), then adjust its size and set its Is Trigger field.

Another possibility is to create a new object to act as the trigger volume, attach the control script to it and link the door to its trigger via a public variable (trigger script):

 var myDoor: Transform; // drag the door here
 private var inRange = false;
 
 function OnTriggerEnter(other: Collider){
   if (other.tag == "Player") inRange = true;
 }
 
 function OnTriggerExit(other: Collider){
   if (other.tag == "Player") inRange = false;
 }
 
 functin Update(){
   if (inRange && Input.GetKeyDown("F")){
     // open/close the door referenced by myDoor
   }
 }