Specific blocked content message per use

Hi there, I have used the php version of the shortcode to hide some content in my theme files and it works great. However I am wondering how to display a custom message for each use of the tag. For example I want one area to say “You must log in to send a message”, and another area say, “This content is locked”.

This is what I used to hide the content:

<?php if ( current_user_can( 'message_agent' ) ) : ?>
 all my content
<?php endif; ?>

How can I add a specific message to people using this format? Also, the default message does not show up either in place of the content for some reason.

Any help is appreciated, thank you.