Trojan service depends on Let's Encrypt TLS certificates obtained via HTTP-01 challenge, which requires port 80 to be accessible for certbot's standalone HTTP server. The landing server's UFW allowed_ports only includes port 443 (Trojan), blocking the Let's Encrypt challenge and preventing certificate issuance.
allowed_ports in group_vars/landing.yml.example for Let's Encrypt HTTP-01 challengegroup_vars/landing.yml.example: add port 80 to allowed_ports