If after you installed osCommerce 4.x you fail to open the admin page or the pages under Sales Channels tab cannot be opened the fact that mod_rewrite does not work may cause this issue.
In order to check that mod_rewrite is enabled and works correctly perform the following actions:
1. Open the root directory (or admin directory if you fail to open the admin page) of your website, create a file check_mod_rewrite.php with the following line:
<?php echo "<h1>Mod_rewrite is working well</h1>";
2. Rename the original file from .htacess to .htacess_original
3. Create a new .htaccess file with the following line:
RewriteEngine On
RewriteRule ^.*$ check_mod_rewrite.php
4. Open your web site (or /admin page). If you see a message “Mod_rewrite is working well”, it is ok. If you see anything else - mod_rewrite is disabled or incorrectly configured.
5. Delete the .htaccess file you created and rename the original file back.