HTTP post request via serverless function in Go returning invalid request error

I'm attempting to write a serverless function in my app, with the function hosted in Vercel.

Whenever I attempt to call the API, I keep getting the following message which is incredibly unspecific:

Checking the actual function logs, I can see the function does get called but it seems to be showing as a GET request, not a POST. Here's a screenshot (partially blanked out):

In my code itself, it doesn't seem like any of it is getting called. The first error doesn't get called for sure, as it isn't printing the defined error message.

What I'm attempting to do is:

• create a new HTTP client
• set a new POST request on the client
• if err at this stage, exit the app
• then add values such as API key etc
• set a header for type of content it accepts
• encode the URL
• 'Do' the call, and log the contents of the response body

However, it's just returning the top screenshot in my browser. Does anyone know what's going wrong?

Here's my function call:

func Fetch(w http.ResponseWriter, r *http.Request) {
	client := &http.Client{}
	req, err := http.NewRequest("POST", "https://auth.truelayer.com/connect/token", nil)

	if err != nil {
		log.Print(err)
		fmt.Println("Error was not equal to nil at first stage.")
		os.Exit(1)
	}

	q := url.Values{}
	q.Add("grant_type", "authorization_code")
	q.Add("client_id", os.Getenv("CLIENT_ID"))
	q.Add("client_secret", os.Getenv("CLIENT_SECRET"))
	q.Add("redirect_uri", "https://url.com/callback")
	q.Add("parameter", req.URL.Query().Get("parameter"))

	req.Header.Set("Accepts", "x-www-form-urlencoded")
	req.URL.RawQuery = q.Encode()

	resp, err := client.Do(req)
	if err != nil {
		fmt.Println("Error sending request to server")
		os.Exit(1)
	}

	respBody, _ := ioutil.ReadAll(resp.Body)
	w.WriteHeader(resp.StatusCode)
	w.Write(respBody)
}

For reference, this is the documentation / API I'm attempting to call: https://docs.truelayer.com/docs/data-api-authentication

You need to pass form values as POST body

client := &http.Client{}

q := url.Values{}
q.Add("grant_type", "authorization_code")
q.Add("client_id", os.Getenv("CLIENT_ID"))
q.Add("client_secret", os.Getenv("CLIENT_SECRET"))
q.Add("redirect_uri", "https://url.com/callback")
q.Add("parameter", req.URL.Query().Get("parameter"))

req, err := http.NewRequest("POST", "https://auth.truelayer.com/connect/token",
	strings.NewReader(q.Encode()))

Set "Content-Type" to "application/x-www-form-urlencoded"

// req.Header.Set("Accepts", "x-www-form-urlencoded")
// req.URL.RawQuery = q.Encode()
req.Header.Add("Content-Type", "application/x-www-form-urlencoded")