Dreamhost 500 Internal Server Error

After I have installed x-cart v4.4.1 under DreamHost for my new domain prescriptionglasses123.com, it shows:
Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, webmaster@prescriptionglasses123.com 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.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I login Putty:
cd prescriptionglasses123.com // enter the folder of precriptionglasses123.com
chmod 755 var //change folder of var to 755
cd var //enter folder of var
chmod 755 . change all folders under var to 755

Done.