Unity Error: Only assignment, call, increment, decrement, and new object expressions can be used as a statement

Questions & Answers
1

This script (ShortRange tower bullet) acquires the monster component of another script (MonsterScript) and takes its health by the amount of damage this bullet does. The bullet is a public float.

The second line of the else method is getting an error (health - damage;)

void OnTriggerEnter(Collider co) {

	if (co.gameObject.tag == "Monster") {  

		if (co.gameObject.GetComponent<MonsterScript> ().health <= 0) { 
			co.gameObject.GetComponent<MonsterScript> ().Death();
			Destroy (co.gameObject);
		}else{
			co.gameObject.GetComponent<MonsterScript> ().myHealthBar.text = co.gameObject.GetComponent<MonsterScript> ().myHealthBar.text.Remove(co.gameObject.GetComponent<MonsterScript> ().myHealthBar.text.Length - 1 ); 
			co.gameObject.GetComponent<MonsterScript> ().health - damage;

		}

Here’s the whole script

public class ShortRangeTowerBullet : MonoBehaviour {

// Speed
public float speed = 10; 

// Target (set by Tower)
public Transform target;  

//Damages the monster 
public float damage;

void FixedUpdate() {

	// Still has a Target?
	if (target) {

		// Fly towards the target        
		Vector3 dir = target.position - transform.position;
		rigidbody.velocity = dir.normalized * speed;
	} else {

		// Otherwise destroy self
		Destroy(gameObject);
	}
} 

void OnTriggerEnter(Collider co) {

	if (co.gameObject.tag == "Monster") {  

		if (co.gameObject.GetComponent<MonsterScript> ().health <= 0) { 
			co.gameObject.GetComponent<MonsterScript> ().Death();
			Destroy (co.gameObject);
		}else{
			co.gameObject.GetComponent<MonsterScript> ().myHealthBar.text = co.gameObject.GetComponent<MonsterScript> ().myHealthBar.text.Remove(co.gameObject.GetComponent<MonsterScript> ().myHealthBar.text.Length - 1 ); 
			co.gameObject.GetComponent<MonsterScript> ().health - damage;

		}
			
	}

}

}

2

You are subtracting damage from health but not assigning it to anything. Try using the -= operator to assign health to health - damage:

co.gameObject.GetComponent<MonsterScript> ().health -= damage;