Plugin Not Found while creating network with IPAM & Driver

[dipak-pawar]

While creating network with following option getting plugin not found error.

networks:
  app_net:
    driver: bridge
    driver_opts:
      com.docker.network.enable_ipv6: "true"
    ipam:
      driver: default
      config:
      - subnet: 172.16.238.0/24
        gateway: 172.16.238.1
      - subnet: 2001:3984:3989::/64
        gateway: 2001:3984:3989::1

We don't have driver_opts in v 1.21.

but I tried it without driver_opts as well, getting the same error.