tender crow
I can't remember too much about axios, but what I do know of it is that response.json() doesn't work for this. I think it needs to be response.data which should have the user information.
If you didn't use the axios configuration to set a base url, then you will need to provide a full url path as your api variable. In other words
const api = "http://localhost:<port>/api/gomoku/generate_username"
This should be the full path to your running server on the specified port.
I also notice that in your route, you are just passing in playerId to res.json but I see no export or module.exports to show that anything is being exported from your module path "../domain/player" and it looks like the create function isn't being used. Hard to say without more code. I also do believe that playerId, if not an object, would technically be invalid json because it looks like you are attempting to send an array. Again, this is just an assumption because I can't see what is being exported from player.js and it looks as though nothing is really being created as far as usernames.
I have no idea if you've hooked up your router properly and passed it the proper way to app.use in your app.js/server.js file either. Or that the server is running.
I would also recommend that you create a post request instead and create/generate a new username. It would be, technically, more semantically correct for creating a resource. A get request should be used in the event the resource is assumed to exist. I don't know that it is proper to create a resource in a get request.
Too many unknowns to be of much help here, but this is the best advice I can give with the information provided. Hopefully it helps.
light forge