use-ipv6 is a partition parameter, not a template parameter.

stack/erp5/buildout.cfg

@@ -294,7 +294,7 @@ md5sum = 9fff70e6958bccbb3d4982cb3611813f
 [template-cluster-zope]
 < = download-base
 filename = instance-cluster-zope.cfg.in
-md5sum = 019f306860857e8536f26aba32101b30
+md5sum = c5c13fb438aa2a4d9ce22880e16fc2cd
 
 [template-balancer]
 < = download-base

stack/erp5/instance-cluster-zope.cfg.in

 {% if slap_software_type == software_type -%}
-{% set use_ipv6 = parameter_dict.get('use-ipv6', False) -%}
+{% set use_ipv6 = slapparameter_dict.get('use-ipv6', False) -%}
 {% set next_port = slapparameter_dict['port-base'] -%}
 {% set site_id = slapparameter_dict['site-id'] -%}
 {% set storage_type = slapparameter_dict['zodb-storage-type'] -%}