On this server does not exist file-type-name 'index.shtml' in DirectoryIndex ,
and user do not have any permission adding the file type 'shtml' in '.hcaccess file'.
If a host user wish to use SSI and let his guests be able to omit typing full file name (index.shtml) after domain name,
(i.e. type 'oebashi.clinic' instead for type full 'www.oebashi.clinic/index.shtml')
this file is nescessary in root directory,
not to be forced to jump into server's default index page.