Issues Encountered with Headless Browser

Your coding is correct,

function BrowserTest()
{
    var browser = Sys.Browser("chrome").FindAllChildren("ObjectType", "BrowserWindow", 1);
        
    for (var i = 0; i < browser.length; i++) {
        browser[i].Activate();
        
        /* This could be an issue*/
        if (NameMapping.Sys.browser.UCLSearchPage.textboxCaseApp.Exists) {
            Log.Message("Found Case App text field in MIUCL page");
        } else {
            Log.Message("Unable to find Case App text field on MIUCL page");
        }
    
        if (browser[i].WndCaption.indexOf("W3Schools") !== -1) {
            Log.Message("Closing Browser Window " + browser[i].WndCaption);
            browser[i].Close();
        }
    }
}

However, the main issue could be relating to the Name Mapping that you are using in the middle of the code. You are referencing browser.UCLSearchPage.textboxCaseApp, but what properties have you defined for the parent browser?