Clicking buttons with Selenium WebDriver Java

There is a button on a website I am testing and Selenium can't seem to click it. I have tried many different ways but still nothing. Attached is the HTML for the button as well as the XPath.
Has anyone else experienced this or knows how to get around this?

XPath:

/html/body/div[1]/div/div/form/div[21]/div[3]/button

[HTML]

Try below code -

Actions action = new Actions(driver);
WebElement My_btn = webdriver.findElement(By.xpath("/html/body/div[1]/div/div/form/div[21]/div[3]/button"));
action.moveToElement(My_btn).click(My_btn).build().perform();

Let me know the outcome.

Could be a synchronization issue. kindly try below solution if your element is not within iframe or else you need to switch control to iframe first before dealing with the web element.

WebDriverWait wait = new WebDriverWait(driver, 30);  
wait.until(ExpectedConditions.visibilityOfElementLocated(By.xpath("/html/body/div[1]/div/div/form/div[21]/div[3]/button"))).click();

Or You can also try javascript solution:

WebElement element= driver.findElement(By.xpath("/html/body/div[1]/div/div/form/div[21]/div[3]/button"));

JavascriptExecutor executor = (JavascriptExecutor) driver;
executor.executeScript("arguments[0].click();", element);