this.template.QuerySelector(...).changeMessage is not a function error in Salesforce. How does this error occur?

 this.template.QuerySelector(...).changeMessage is an error that may occur when you try to execute the changeMessage method on the div element. You can add the child component to the markup of the parent and write your code like the following one: Const childCompinent = this.template.QuerySelector(‘c-child-cmp’);
If (childComponent) {
    childComponent.changeMessage(event.target.value);
}

This error may also occur as the querySelectorAll differs from the querySelector, and it returns a node list of elements. It means you try to call a method from a list of elements.