Strange error on Xlworkbook line

Question

Personal I appeared this message in my code that I am posting in the image, someone can help me in search of a solution;

Follows the code:

protected void importar_Click(object sender, EventArgs e)
{
    if (selecionarArquivo.HasFile)
    {
        if (Path.GetExtension(selecionarArquivo.FileName) != ".xlsx" && Path.GetExtension(selecionarArquivo.FileName) != ".xls")
            ClientScript.RegisterStartupScript(typeof(string), "Erro", "<script>alert('Somente arquivos em excel')</script>");
        else
        {
            if (selecionarArquivo.FileContent != null)
            {

                var wb = new XLWorkbook(selecionarArquivo.FileContent);
                var planilha = wb.Worksheet(1);

                var linha = 1;
                while (true)
                {
                    //aqui vc vai pegar os campos que deseja atrave da especificação da Coluna e da Linha
                    var nome = planilha.Cell("A" + 6.ToString()).Value.ToString();
                    break;
                }

                var cont = 1;
                //var ListaAplImportProcesso = new List<AplImportProcesso>();
                foreach (var x in planilha.RowsUsed())
                {
                    ListaAplImportProcesso.Add(new AplImportProcesso
                    {
                        dataDeRequisicaoPgto = Convert.ToDateTime.Cell(cont),
                        NumeroProcessoAnterior = x.Cell(cont + 1).GetString()
                    });
                    cont++;
                }
            }
        }
    }
}

Answer 1

Since you did not install the package then you do not have the appropriate Dlls. By not having these Dlls you will get the specified error

Type or namespace 'Xlworkbook' could not be found.

In Solution Explorer > References > Right click on mouse > Add Reference > COM Menu > Search for Microsoft Excel 16.0 Object Library (or smaller depending on your Excel).