Classical: you forgot the closing brace of setup(). (or of the for loop at its end, judging by the indentation).

Thankyou for reply. yes you are right. I missed the closing brace. and now the error is gone. Thankyou.

May I ask you one more question?

I cant figure out that when I run this, the error says that "Cannot invoke dragPressed() on the array type MainPanel[]" on the line ::circle.dragPressed() under the 'void mousePressed()'.

What should I do? I tried adding [i] at the end of circle but it did not work.

First, I find the declaration strange: Memory[] circle = new Circle[numCircle]
Since I see no Memory declaration and Circle doesn't extend it.
And indeed, circle (I would name it circles, to show it represents multiple objects) is an array, and arrays in Java doesn't have such method. You tried adding [i], but if you don't declare i, you would get a syntax error.
Actually, you need to loop on the circles (for loop) and call the method on each instance.

>Memory[] circle = new Circle[numCircle],  I copied the wrong one.

Im not sure what you mean by this, "circle (I would name it circles, to show it represents multiple objects) is an array, and arrays in Java doesn't have such method." sorry about my English.

Here now I managed to display three circles but cant drag them. the error says ArrayIndexOutBoundsException: 3

-----------------------------------------------------------------------------------------------------------------------------

final color BACKGROUND = color (0, 0, 0) ; // the background colour(BLACK)

int numCircle = 3 ; // the number of memory
Circle[] circles = new Circle[numCircle] ; // declare and create the array
// -------------------------------------------------
void setup() {
  size(960, 600) ; // set the mianpanel size as 960 * 600)
  background(BACKGROUND) ; // set the background colour
  smooth() ;
  noStroke() ;

  for (int i = 0; i < circles.length; i++) {
    // set the position of the circles (in this case, random)
    float x = random(100, 700) ;
    float y = random(300, 600) ;
    circles[i] = new Circle(x, y, 50) ; // create each object
  }
}

// ------------------------------------------------------------------------------ Main methods
// Display the elements
void draw() {
  background(BACKGROUND) ;
  for (int i = 0; i < circles.length; i++) {
    circles[i].paint() ; // draw the objects consist in the Memory paint() function
  }
}
// -------------------------------------------------
void mousePressed() {
  circles[numCircle].dragPressed() ; // when the mouse is pressed, do the drawPressed() function
}
void mouseReleased() {
 circles[numCircle].dragReleased() ; // when the mouse is released, do the drawReleased() function
}
---------------------------------------------------------------------------------------------------------------------------------

class Circle {
  // ----------------------------------------------------------------- Constants and variables
  private float x, y ; // the x and y-coordinate of the circle(object)
  private float dx, dy ; // the difference between the circle's position and the mouse position
  private int radius ; // the radius of the circle
  private boolean startDrag ; // prepare for drag signal
  // ---------------------------------------------------------------------------- Constructors
  Circle(float xPos, float yPos, int radi) {
    this.x = xPos ;
    this.y = yPos ;
    this.radius = radi ;
  }
  // --------------------------------------------------------------------------- Public methods
  public void paint() {
    // when the mouse drag starts, starDrag turns "true" and implement the function
    if (startDrag == true) {
      x = mouseX + dx; 
      y = mouseY + dy;
    }
   
    // Draw the circle
    fill(255) ;
    for (int i = 0; i < numCircle; i++){
    ellipse(x, y, radius, radius) ;
    }
  }
  // ------------------------------------------------------- Getters and setters
  // ------------------------------------------------------- Setters
  // ------------------------------------------------------ Other public methods
  public void dragPressed(){
    // if the centre of the cicle and the mouse position is less than the radius of the circle,
    if (dist(x, y, mouseX, mouseY) <= radius) {
      startDrag = true ; // the drag turns "true"
      // calculate the distance between the circle's position and that of mouse
      dx = x - mouseX;
      dy = y - mouseY;
    }
  }
 
  public void dragReleased(){
    startDrag = false ; // the darg turns "false"
  }
  // -------------------------------------------------------------------------------------------
}
// -------------------------------------------------------------------------------------------

 

circles[numCircle].dragPressed();
numCircle is the size of the array. Indexes in arrays go from 0 to numCircle - 1

Try:
void mousePressed() {
  for (int i = 0; i < numCircle; i++) {
    circles[i].dragPressed() ; // when the mouse is pressed, do the drawPressed() function
  }
}
and same for the other function.

Cool! It worked. Thankyou very much for your help!