Unable to generate GRPC interface for Python?

[JaydenFish]

Do I have to use protobuf with grpc? flatc version 24.3.25 with a simple fbs:

namespace models;

table HelloReply {
  message:string;
}

table HelloRequest {
  name:string;
}

table ManyHellosRequest {
  name:string;
  num_greetings:int;
}

rpc_service Greeter {
  SayHello(HelloRequest):HelloReply;
  SayManyHellos(ManyHellosRequest):HelloReply (streaming: "server");
}

and run flatc -c -p --grpc -o api/ include/fbs/greeter.fbs output is

error:
  Unable to generate GRPC interface for Python