Unable to click on an object when using GetElementsByClassName

What's the contents of that one item in the array?  While it's true that the array may have a single element, it could be that it's a null value or simply an empty element.  The GetElementsByClassName is not a TestComplete method so it's hard to say what it returns.

This is why I suggested FindAllChildren.  What happens if you change your code to

function RelVis(){
var page = browser.CH;
//this return an array of all the object found with the following criteria 
var buttonRel = page.FindAllChildren("className", "btn acute-btn rel", 100);

//Log.Message(buttonRel.length);

if (buttonRel.length > 0) then {

    if(buttonRel[0].Exists){
      buttonRel[0].Click();
    }
}
  }