Release or Environment
In order to use the Flow Designer with Service Catalogs, the Flow Designer support for the Service Catalog [com.glideapp.servicecatalog.flow_designer] plugin needs to be activated.
For more details see documentation:
When the plugin is successfully activated, then configure the Catalog Item record form and add the Flow field to the form.
For information about How to create a Flow with a Service Catalog trigger, see documentation: