It looks like your ID,
comments-task-6-59ba23e91bb06
is dynamically generated. I suggest you use regular expression in your XPath expression.
Something as given below.
wait.until(ExpectedConditions.elementToBeClickable(By.xpath("//*[@id=\"^comments-task*\"]/div/div/div[3]/div[2]/form/button")));
The above may not work as I just tried to give you an example. You can find regular expression by googling it out.