If you have just created an account or addon domain and try viewing it in a browser you may see a 403 error.
To find out more information you will want to check the Apache error_log file.
If you see an entry saying Cannot serve directory this is due to the absence of any matching file for Apache to server and the DirectoryIndex being disabled. The error will look similar to this.
[Mon May 17 02:12:02.952827 2021] [autoindex:error] [pid 8506:tid 47719184406272]
[client 220.127.116.11:49999] AH01276: Cannot serve directory
/home/username/public_html/: No matching DirectoryIndex (index.php,index.php8
and server-generated directory index forbidden by Options directive
To correct this you would want to make sure you have uploaded your website's file. Most commonly an index.html or index.php file.
If you have a custom-named file you want to be loaded instead of those listed files, you will need to add it to the Apache configuration. To do so you can log into WHM and click on Apache Configuration, then click DirectoryIndex Priority. From there you can add the new filename or re-order the list if you use multiple files that match the names in the list.
For more information on the DirectoryIndex Priority see the following documentation.