How to locate the Constraint validation text using Selenium and Java

I'm trying to make a Selenium test script that checks if a bootstrap validation popover appears when submitting a form containing a bad value.

My script below returns this error:

org.openqa.selenium.NoSuchElementException: no such element: Unable to locate element: {"method":"css selector","selector":"#amount"}

Relevant code:

WebDriver driver = new ChromeDriver()
WebElement field = driver.findElement(By.id("amount"));  //errors here every execution
Boolean is_valid = (Boolean)WebUI.executeScript("return arguments[0].checkValidity();", field);

if (!is_valid) {
     //intentionally fail test
}

When I inspect the form field, I see the id equals amount, so why am I unable to find this element in Selenium?

related question on SO

EDIT Here's my full script that I omitted for brevity:

WebUI.openBrowser('')

WebUI.navigateToUrl('Foo')

WebUI.setText(findTestObject('Object Repository/Page_bar/input_concat(Recipient, , s email address)_email'), 'fakepersonaluser1@example.com')

WebUI.setText(findTestObject('Object Repository/Page_bar/input_Amount (USD)_amount'), '10001')

WebUI.click(findTestObject('Object Repository/Page_bar/button_Send Payment'))

 
WebDriver driver = new ChromeDriver() WebElement field =
driver.findElement(By.id("amount")); Boolean is_valid =
(Boolean)WebUI.executeScript("return arguments[0].checkValidity();", field);

if (!is_valid) { //intentionally fail test }

WebUI.closeBrowser()