Basic custom Shortcode not working

get_the_content returns the content as is without processing, it does not pass it through the filters that are used in the_content such as oembeds or shortcodes. This filter is also called the_content ( coincidence )

Instead use this:

$email_body = apply_filters( 'the_content', get_the_content() );

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)