Do you have these in your showthread template? Did you upgrade from which version or did you install first time?
<script type="text/javascript" src="{$mybb->settings['bburl']}/jscripts/thankyoulike.min.js"></script>
<script type="text/javascript">
<!--
var tylEnabled = "{$mybb->settings['g33k_thankyoulike_enabled']}";
var tylDisplayGrowl = "{$mybb->settings['g33k_thankyoulike_displaygrowl']}";
var tylCollapsible = "{$mybb->settings['g33k_thankyoulike_collapsible']}";
var tylUser = "{$mybb->user['uid']}";
var tylSend = "{$lang->tyl_send}";
var tylRemove = "{$lang->tyl_remove}";
// -->
</script>
[MyBB 1.8 Czech translation] [MyBB 1.8 plugins]: Prune old PMs + optimize DB plugin --- Thank you/like system