Mixin that extends jnt:category not available in categories
Customer extends jnt:category with a mixin similar to
[jmix:extend_category] mixin extends = jnt:category itemtype = content - name (string) mandatory
but the mixin is not available on categories
Categories are managed by system site. As such it is necessary to enable the module on system site for the mixin to be available.
For enabling the module, from admin UI, go to system components --> Modules, go into details of your module and enable it on system site under 'Usage in sites'.