The error message "The template does not support the debtor's language" means the debtor has a default language set in iController for which no template was created yet.
iController will always match the language of your debtor with the corresponding translation of the required template.
E.g. if your debtor's default language is French, you will need to create a French version of the template you need.
View our related article: How do I create translated versions of a template?