Http Post reponse return nil

Getting Help
1

Hello Community.
I was trying to get the token on Paypal with a http (module net/http) Post request.
The link to example: example
I tried decode and unmarshall and any way found on the internet.

respBody, err := ioutil.ReadAll(resp.Body)
json.Unmarshal([]byte(respBody), &info) // inf == []

json.NewDecoder(resp.Body).Decode(&res) // res == nil

I could see the bytes as int or the string converted but not the json struct.
I tried to use every of these:

var res interface{}
var res []interface{}
var res map[string]interface{}

I am new to golang.