There are two possible reasons why 500 Internal Server Error might appear.
Some webservers do not allow executing PHP scripts with the permissions higher than 755 or 644. Change the permissions for your files and directories to 755 in order to resolve this issue.
To set up the permissions, complete the steps below:
chmod 755 index.php
The command sets the 755 permissions for all CS-Cart scripts (at least, for index.php and admin.php located in the CS-Cart root directory, and for index.php located in the install directory). Use this command, if you get the mentioned 500 Internal Server Error after the installation.
The 755 permissions mean read, write and execute permissions for the file user, and read and write permissions for the user group and others. Learn more about permissions in CS-Cart documentation.
If you do not have terminal access to your server, change file access permissions using an FTP client. Most FTP clients have the Change permissions or Change mode functions, that you can use to set permissions. For more detailed instructions on how to set file permissions refer to your FTP client documentation.
Contact your hosting support team for information about proper permissions for PHP files and directories.
The second possible reason for 500 Internal Server Error is that your web server does not support some directives of the CS-Cart .htaccess file.
To solve the problem, try renaming the .htaccess file located in the root directory of your CS-Cart installation to something else, for example, .htaccess1.
If renaming .htaccess helps, contact your server administrator. Send them the content of the CS-Cart .htaccess file and ask which directives are not supported on your server.