C # conversion error

I have the most generic class Account and another ContaPoupaça that inherits from this class Conta . In this daughter I have an interface called ITributavel , because this class in addition to the methods and attributes of Conta , has a ITributavel method.

I have another class TotalDeTributos that has a method that does an operation with class ContaPoupanca that has ITributável . In the AdicionaConta method, of the photo below, I put as argument count of type Conta and it is giving this error, I already tried to use casting to convert novaConta but gives another error. >

I do not want the argument to count as type ContaCorrente , because other classes that inherit from the Account will have ITributável and another not.

What am I doing wrong?

Form1:

using System;
using System.Windows.Forms;

namespace Banco2
{
    public partial class Form1 : Form
    {

        int indice1 = 0;
        int indice2 = 0;
        private int indiceNovaConta = 0;
        string tipoConta = "";

        private Conta[] contas = new Conta[10];
        TotalDeTributos tributos;
        ContaPoupanca c1;
        ContaInvestimento c2;
        Conta selecionada;



        public Form1()
        {
            InitializeComponent();
        }


        private void Form1_Load(object sender, EventArgs e)
        {
            c1 = new ContaPoupanca();
            c1.Titular = new Cliente("Rodrigo");
            c1.Numero = 1;
            this.AdicionarConta(c1);



            c2 = new ContaInvestimento();
            c2.Titular = new Cliente("Diego");
            c2.Numero = 2;
            this.AdicionarConta(c2);


            tributos = new TotalDeTributos();


            AtualizaTributo();
            lblValor.Text = "0";
        }

        private void btnDeposita_Click(object sender, EventArgs e)
        {
            double valorAcao = Convert.ToDouble(lblValor.Text);

            contas[indice1].Deposita(valorAcao);
            MessageBox.Show("Deposito realizado.");
            txtSaldo.Text = Convert.ToString(contas[indice1].Saldo);
            lblValor.Text = "0";
            AtualizaTributo();

        }

        private void btnSaque_Click(object sender, EventArgs e)
        {
            double valorOperacao = Convert.ToDouble(lblValor.Text);

            contas[indice1].Saque(valorOperacao);

            txtSaldo.Text = Convert.ToString(contas[indice1].Saldo);
            lblValor.Text = "0";

        }

        private void btnTransferir_Click(object sender, EventArgs e)
        {
            double valorOperacao = Convert.ToDouble(lblValor.Text);
            contas[indice1].Transfere(valorOperacao, contas[indice2]);

            txtSaldo2.Text = Convert.ToString(contas[indice2].Saldo);
            txtSaldo.Text = Convert.ToString(contas[indice1].Saldo);
            lblValor.Text = "0";

        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            indice1 = comboBox1.SelectedIndex;
            selecionada = this.contas[indice1];

            txtTitular.Text = selecionada.Titular.Nome;
            txtSaldo.Text = Convert.ToString(selecionada.Saldo);
            txtNumConta.Text = Convert.ToString(selecionada.Numero);

        }

        private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
            indice2 = comboBox2.SelectedIndex;
            selecionada = this.contas[indice2];

            txtTitular2.Text = selecionada.Titular.Nome;
            txtSaldo2.Text = Convert.ToString(selecionada.Saldo);
            txtNumConta2.Text = Convert.ToString(selecionada.Numero);

        }

        public void AdicionarConta(Conta novaConta)
        {


            if (novaConta is ContaPoupanca)
            {
                this.tipoConta = "Conta Poupança";
            }
            else if (novaConta is ContaCorrente)
            {
                this.tipoConta = "Conta Corrente";
            }

            this.contas[this.indiceNovaConta] = novaConta;
            Conta selecionada = this.contas[this.indiceNovaConta];
            comboBox1.Items.Add(selecionada.Titular.Nome + " - Tipo: " + this.tipoConta);
            comboBox2.Items.Add(selecionada.Titular.Nome + " - Tipo: " + this.tipoConta);

            if(selecionada is ContaPoupanca)
            {
                MessageBox.Show(selecionada.GetType().ToString());
                tributos.Acumula((ITributavel)selecionada);
            }

            indiceNovaConta++;

        }

        private void btnCadastrar_Click(object sender, EventArgs e)
        {
            FormCadastrarConta formCadastrarConta = new FormCadastrarConta(this);
            int indiceUltimaConta = contas.Length;
            formCadastrarConta.ShowDialog();
        }

        public void AtualizaTributo()
        {

            if (tributos != null)
            {
                foreach(Conta conta in contas)
                {
                    if(conta is ContaPoupanca || conta is ContaInvestimento)
                    {
                        tributos.Acumula((ITributavel)conta);
                    }
                }


                txtTributos.Text = Convert.ToString(tributos.Total);
            }

        }

    }
}

TotalTymbols:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Banco2
{
    class TotalDeTributos
    {
        public double Total { get; private set; }

        public void Acumula(ITributavel conta)
        {
            this.Total += conta.CalcularTributo();
        }

    }
}

Taxable:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Banco2
{
    interface ITributavel
    {
        double CalcularTributo();
    }
}