index out of band when trying to select a message using protractor

I am trying to select a warning message "unassigned amount.." with this HTML code HTML code for warning message

I tried to select it using this line of code

  var warning=element.all(by.xpath('.//span[.="Unassigned Amount = 20.00"]')).filter(function (elm) {
    return elm.isDisplayed().then(function (isDisplayed) {
        return isDisplayed;
    });
});

but I got this error from protractor : enter image description here