SQL Connection Test (Error)

static class ConnectionManager
    {
        public static string Username { get; set; }
        public static string Password { get; set; }
        public static string Database { get; set; }
        public static string Server { get; set; }
        public static SqlConnection Connection { get; set; }

        private static string ConnectionString { get { return $@"Data Source={Server};Initial Catalog={Database};Integrated Security=SSPI;User ID={Username};Password={Password};"; } }

        private static bool _ready = false;

        /// <summary>
        ///
        /// </summary>
        /// <param name="user">Name of the user (with domain)</param>
        /// <param name="pass">Password of the user</param>
        /// <param name="database">Name of the database</param>
        /// <param name="server">Name of the server</param>
        public static void Initialize(string user, string pass, string database, string server)
        {
            try
            {
                /*
                Username = user;
                Password = pass;
                Database = database;
                Server = server;

                Connection = new SqlConnection(ConnectionString);
                */

                Connection = new SqlConnection(new SqlConnectionStringBuilder()
                {
                    DataSource = server,
                    InitialCatalog = database,
                    UserID = user,
                    Password = pass
                }.ConnectionString);

                Connection.Open();

                _ready = true;
                Console.WriteLine("A conexão foi aberta.");
            }
            catch(SqlException sqlex)
            {
                Console.WriteLine(sqlex.Message);
            }
            finally
            {
                _ready = false;
            }
        }

        /// <summary>
        ///
        /// </summary>
        /// <param name="tableName"></param>
        /// <param name="fields">Example: field1,field2,field3,field4</param>
        /// <param name="values"></param>
        public static void Insert(string tableName, string fields, params string[] values)
        {
            if(values.Length > 0 && _ready)
            {
                try
                {
                    using (SqlCommand comm = new SqlCommand($"INSERT INTO {tableName} VALUES ({fields})", Connection))
                    {
                        for (int i = 0; i < values.Length; i++)
                        {
                            comm.Parameters.AddWithValue($"@{fields.Split(',')[i]}", $"@{values[i]}");
                        }

                        comm.ExecuteNonQuery();
                        Console.WriteLine($"{values.Length} campos foram inseridos.");
                    }
                }
                catch(SqlException sqlex)
                {
                    Console.WriteLine(sqlex.Message);
                }
                finally
                {
                    Connection.Close();
                }
            }
            else
            {
                Console.WriteLine("Nenhum parametro foi inserido ou conexão não está pronta.");
            }
        }

        public static void EndConnection()
        {
            Connection.Close();
            Connection.Dispose();
        }

        public static void ClearParameters()
        {
            Username = "";
            Password = "";
            Database = "";
            Server = "";
        }
    }