Cannot get character to move using on-screen controls

Question

This problem that I am having is related to a 2D game that I am working on. All of the other questions that I have seen asked are related to the Unity3D versions of games and tutorials. I don't understand why it isn't working for me.

In my script, I am trying to get my player to move left and right. It is strange to me because I can get the player to jump, fall down, and throw a snowball using the buttons that I created on the screen. For some reason, however, the player will not respond to the left button clicks and right button clicks that I placed on the screen. Any help is greatly appreciated, thank you in advance.

 void Update () {
 if (Input.GetKey(left))
 {
     MoveLeft();
 }
  else if (Input.GetKey(right))
 {
     MoveRight();
 }
 else
 {
     StopMoving();
 }

public void MoveLeft()
{
 theRB.velocity = new Vector2(-moveSpeed, theRB.velocity.y);
}
public void MoveRight()
{
 theRB.velocity = new Vector2(moveSpeed, theRB.velocity.y);
}
public void StopMoving()
{
 theRB.velocity = new Vector2(0, theRB.velocity.y);
}

EDIT:

This is what you have done:

if (CrossPlatformInputManager.GetAxis("Horizontal")>0){
    MoveRight();
}

else if (CrossPlatformInputManager.GetAxis("Horizontal")<0){
    MoveLeft();
}

else{
    StopMoving();
}

Event better, you can pass the speed to the MoveXXX functions and multiple the value by moveSpeed.

Example:

float horiAxis = CrossPlatformInputManager.GetAxis("Horizontal");
if (horiAxis > 0){
    MoveRight(horiAxis);
}

public void MoveLeft(float val)
{
    theRB.velocity = new Vector2(-moveSpeed * val, theRB.velocity.y);
}