Thank you for your answer, unfortunately I just get a white page :/
This is the full line that causes problem :
if(has_capability('moodle/calendar:managegroupentries', $context)) {echo "<li><a href='".$CFG->wwwroot."/report/completion/?course=10&group=1'>".$USER->lang == 'es' ? 'Informes' : 'Reports'."</a></li>";}?>
This works well if I replace has_capability by if(is_siteadmin()), but that doesn't do the trick for what I need to do unfortunately