This is often from keeping around a reference to a dead transform. For example, you cache some component, transform, or gameobject of an enemy that is being attacked by the player. But then it dies. When this reference is encountered again, it will report that it is dead - this error. The solution is to check to make sure that whatever thing you’re about to access is not dead. This could be done by adding a bool isDead.
if(!someThing.isDead)
DoStuff(); // attack it until it is dead
else DoOtherStuff(); // find different thing to attack
When something is killed, set isDead = true.
i would reccomend doing it that way:
If(yourObject !=null) // “yourObject” - object you are referencing,
{
//your script here;
}