On my website when I click on the product add to cart button then its display "HTTP ERROR 500" display.
If you are not sure where to find the web server logs (the error details are there), open index.php file and uncomment the following line
So, you will have in result
Save the file and try the add to cart operation once again. You should get the error details on your screen. Do not forget to comment the mentioned line once you have the error details, especially on the production environment.