[Vuejs]-Why is my post request returning a 405 error?

You have to define which methods can be used by the remote origin with the ‘Access-Control-Allow-Methods’ header, example:

Access-Control-Allow-Methods: OPTIONS, HEAD, GET, POST

See https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS

I think is better if you use the firebase Lib from npm. REST API of firebase Database is extrem ineficcent because it downloads many unuseful data and you need to keep in mind that firebase bills the amount of downloaded data. But you don’t specify which module you are using. It would be also great if you post the code.