Webelement input not interactable (when choosing a specific one from list)

I'm trying to find a specific input and sendkeys to it on this website. I have no problem locating it in the browser's console with for example (//input)[4] . However, when I'm trying to find it and sendkeys in my IDE (using Java with Selenium) I got the org.openqa.selenium.ElementNotInteractableException: element not interactable* exception. I tried to do it with following methods:

1. With simple findelement and sendkeys:

   WebElement ebookAddInput = driver.findElement(By.xpath("(//input)[4]"));
   ebookAddInput.clear();
   ebookAddInput.sendKeys("4");
   

2. Using list of items to choose specific one:

   List<WebElement> allInputs = driver.findElements(By.xpath("//input[@name='commerce-add-to-cart-quantity-input']"));
   System.out.println(allInputs.size());
   allInputs.get(4).sendKeys("4");
   

In both cases I got the mentioned error. The only way it works is through not specifying which input i want to choose and then there is no error and first input on the website gets filled:

WebElement ebookAddInput = driver.findElement(By.xpath("//input[@name='commerce-add-to-cart-quantity-input']"));
ebookAddInput.clear();
ebookAddInput.sendKeys("4");

Does someone know why is that happening and if theres way to solve it? Will really appreciate the help.