There is typo in variable name in:

https://github.com/willianmano/moodle-block_ranking/blob/master/lang/en/block_ranking.php

There is

$st*ir*ng['graph_access_deny']
$st*ir*ng['graph_no_groups']

instead of

$st*ri*ng['graph_access_deny']
$st*ri*ng['graph_no_groups']