@Ahjira @stevenkword
As Suzanne mentioned, the bp_message_thread_delete_link() functions are only for the message index. On single message pages, the $thread_template global is used in place of the $messages_template global, and the usable template tags all begin with bp_the_thread_ instead of bp_message_thread_. While there is no bp_the_thread_delete_link() function, it’s pretty easy to just directly add the code from the bp_get_message_thread_delete_link() function with appropriate substitutions.
First, at the top of the members/single/messages/single.php template file, add this line:
<?php global $thread_template, $bp; ?>
Then, wherever you want to add a delete link, add something like the following:
<a href="<?php echo apply_filters( 'bp_get_message_thread_delete_link', wp_nonce_url( $bp->loggedin_user->domain . $bp->messages->slug . '/inbox/delete/' . $thread_template->thread->thread_id, 'messages_delete_thread' ) ); ?>" title="Delete this message" class="button">Delete</a>
the <?php ?> code block above contains the adapted version of the bp_message_thread_delete_link() function.