Quote Originally Posted by Shan90 View Post
Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [no address given] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.
That message instructs you to contact the server administrator. That would be your hosting company.

What did they say?

Additional related FAQ: https://www.zen-cart.com/tutorials/index.php?article=63

Worrying about DIR_FS_CATALOG is irrelevant at this stage.