Why won't this submenu page show? - My First WordPress Plugin
Solution 1:
You are missing the thrid menu_title argument, and you also have a typo in the manage-options. Should be manage_options
add_submenu_page(
'core-settings-page',
'Code Snippets',
'Code Snippets',
'manage_options',
'core-settings-code-snippets',
'core_settings_code_snippets_markup'
);
