What I found is
the function was defined in wp-admin/includes/plugin.php as mentioned here https://wpseek.com/function/wp_add_privacy_policy_content/
When I opened the plugin.php file of a clean wordpress 4.9.8 version I found the function wp_add_privacy_policy_content() already defined, whereas in my wordpress 4.9.8 version It was missing so I added it back
Kindly make sure the function is defined in plugin.php
/**
* Helper function for adding content to the Privacy Policy Guide.
*
* Plugins and themes should suggest text for inclusion in the site's privacy policy.
* The suggested text should contain information about any functionality that affects user privacy,
* and will be shown on the Privacy Policy Guide screen.
*
* A plugin or theme can use this function multiple times as long as it will help to better present
* the suggested policy content. For example modular plugins such as WooCommerse or Jetpack
* can add or remove suggested content depending on the modules/extensions that are enabled.
* For more information see the Plugin Handbook:
* https://developer.wordpress.org/plugins/privacy/suggesting-text-for-the-site-privacy-policy/.
*
* Intended for use with the `'admin_init'` action.
*
* @since 4.9.6
*
* @param string $plugin_name The name of the plugin or theme that is suggesting content for the site's privacy policy.
* @param string $policy_text The suggested content for inclusion in the policy.
*/
function wp_add_privacy_policy_content( $plugin_name, $policy_text ) {
if ( ! is_admin() ) {
_doing_it_wrong(
__FUNCTION__,
sprintf(
/* translators: %s: admin_init */
__( 'The suggested privacy policy content should be added only in wp-admin by using the %s (or later) action.' ),
'<code>admin_init</code>'
),
'4.9.7'
);
return;
} elseif ( ! doing_action( 'admin_init' ) && ! did_action( 'admin_init' ) ) {
_doing_it_wrong(
__FUNCTION__,
sprintf(
/* translators: %s: admin_init */
__( 'The suggested privacy policy content should be added by using the %s (or later) action. Please see the inline documentation.' ),
'<code>admin_init</code>'
),
'4.9.7'
);
return;
}
if ( ! class_exists( 'WP_Privacy_Policy_Content' ) ) {
require_once( ABSPATH . 'wp-admin/includes/misc.php' );
}
WP_Privacy_Policy_Content::add( $plugin_name, $policy_text );
}