IF YOU ARE USING A CUSTOM TEMPLATE NOT BASED ON THE SIX TEMOLATE YOU HAVE TO MODIFY THE CODE SO IT SUIT YOUR CUSTOM TEMPLATE

Delete the file name viewticket.tpl from this directory "modules/addons/ReportTicket/templates/client"

In your template folder look for viewticket.tpl file open the file with your favorite editor and add the code below to your template

{if $reply.id && $reply.admin && $enable_report}

<button type="button" data-ticketid="{$tid}" data-replyid="{$reply.id}" class="btn btn-sm btn-warning reportTicketBtn">{$_RTRLang.client.reportthisreply}</button>

{/if}