Did you post full code ? It seems that you created map "Users" (capitalized) but you are trying to access that map with lowercase "users" in main.go. Maybe I'm missing something? Also, maybe don't use same name for struct and map (name it UserMap or something similar) to avoid confusion.

var UserMap = map[string]*User{
    "john": {Username: "john", Password: "123", Secret: ""},
}

user, ok := users.UserMap[username]