The sh404SEF® documentation are only guidelines. There is no one-size-fits-all instructions about the content of a .htaccess file, as it must be tailored to your specific server settings. As a reminder :
Start with Joomla-supplied .htaccess file. It is in your web site root directory, currently called htaccess.txt. You must rename it to .htaccess file for it to become active. Before you rename it to .htaccess, this file has absolutely no effect whatsoever. For use with sh404SEF®, as with other SEF components, you need to make a few changes inside this file (best to do them before renaming it to .htaccess). Please read documentation again.
If this does not work, you should contact your hosting company to assist in the process, they are the ones who know about how your server is configured.
Please note that if you are using IIS (Microsoft Internet Information Server), rather than Apache, then using a .htaccess file requires installing additional software, as .htaccess are not a built-in feature of IIS. In such cases, the preferred option is to use only the "no .htaccess (/index.php/)" or "no .htaccess (/index.php?/)" options in sh404SEF® configuration, in Advanced Tab.