You have 3 errors

Error 1
change the line

  myBus = new Bus;
to
  myBus = new Bus();

Error 2
change the line
  bus(xPos, yPos) {
to
 Bus() {
This is the constructor for the class and the constructor name MUSt be the same as the class (case senitive)
xPos and yPos are attributes of the Bus class so I got rid of them here

Error3
You don't have a } to match the { at the beginning of the Bus class so add a } at the end of the sketch.

Just in case I have added the corrected program below - it worked for me.

1. Bus myBus;
2. void setup() {
3.   size(500,500);
4.   background(255);
5.   myBus = new Bus();
6.   smooth();
7. }
8. void draw() {
9.   background(255);
10.   myBus.display();
11.   myBus.drive();
12. }
13. class Bus {
14.   int xPos;
15.   int yPos;
16.  
17.   Bus() {
18.     xPos=0;
19.     yPos=height/2;
20.   }
21.  
22.   void display() {
23.     fill(255,0,0); //drawing the bus
24.     stroke(0);
25.     rect(xPos,yPos,100,40);
26.     fill(0);
27.     ellipse(xPos+25,yPos+40,20,20);
28.     ellipse(xPos+75,yPos+40,20,20);
29.   }
30.   void drive() {
31.     xPos++; //moving the bus to the right
32.     if(xPos>width) {
33.       xPos= width;
34.     }
35.   }
36. }