Cannot find button element on form using Webdriver - selenium-webdriver

Question

I'm having trouble trying to click on on a button within a form.
I've tried xpath, cssselector, className, id, but still cannot find it.
Here's the HTML snippet for the button:
<input type="button" value="Continue" id="ni-reg-btn-register" class="btnNext ni-reg-btn-register">
I'm using WebDriver in Java
Getting this trace:
org.openqa.selenium.ElementNotVisibleException: Element is not currently visible and so may not be interacted with
Command duration or timeout: 30.09 seconds
Frequency: 100%
Browser: Firefox
URL: https://subscription.thetimes.co.uk/webjourney/webj_capturecustomerdetails
I've tried each of the following lines of code one by one (but no luck):
driver.findElement(By.className("btnNext ni-reg-btn-register")).click();
driver.findElement(By.cssSelector("buttons#ni-reg-btn-register.btnNext ni-reg-btn-register")).click();
List<WebElement> buttonlist= driver.findElements(By.className("btnNext ni-reg-btn-register"));
driver.findElement(By.id("ni-reg-btn-register")).click();
driver.findElement(By.xpath("//*[#id="ni-reg-btn-register"]")).click();

Answer 1

I was able to click the Continue Button using the below xpath.
//input[#id='ni-reg-btn-register']

Answer 2

I would suggest trying a WebDriverWait call to wait for the element in question to exist on the page prior to interacting with it. The Java documentation can be found here. It appears that while you are using the correct locator and your page is working, there is a timing issue where Selenium is trying to access the element prior to it being loaded.
EDIT: I was unable to locate an element on the provided page with the supplied id. I assume I am missing a step, but I did a search on the HTML for that page and found nothing.

Answer 3

I dont find the above button on the page link you provided ..
better chk the page link and try to give some wait statement after page load ...
implicit wait would be better or simpli Thread.sleep(2000);