A commercial Vaadin subscription is required to use Polymer Templates in your project.
Polymer support has been deprecated since Vaadin 18 (released in November 2020), in favor of faster and simpler Lit templates. The built-in support for Polymer templates has been removed and is only available as a commercial add-on. However, a free conversion tool is available to assist in converting Polymer templates to Lit.
Read more about setting up the commercial Polymer templates addon in the Upgrade Guide.