hi,
The htaccess allows for restricting the access of the website. If you only allow the access of your website by a username/password, like some people do when developing a new website, PayPal's notification server won't know these and thus won't be able to send the payment notification.
But that's not the case on your website, so you can ignore that point.
The most common causes, come from the settings of the PayPal payment method.
So if, while checking the settings there based on what is written in the documentation, you didn't find a problem with them, please provide a screenshot of these settings so that we can double check.
If the settings there are fine, then you can activate the debug setting of the paypal payment method, do a test payment and check the "payment log file" of the HikaShop configuration after the payment. It should contain information about the payment notification.
Also, PayPal has a payment notification history area in your PayPal merchant account which will give valuable information on the payment notification so you can check there for any information.