How to Solve external table is not in expected format error

string conStr = "";
switch (fileExtens)
{
    case ".xls": //Excel 97-03

        conStr = ConfigurationManager.ConnectionStrings["Excel3ConString"].ConnectionString;
        break;

    case ".xlsx": //Excel 07

        conStr = ConfigurationManager.ConnectionStrings["Excel7ConString"].ConnectionString;
        break;
}
conStr = String.Format(conStr, locat);


OleDbConnection connExcel = new OleDbConnection(conStr);
OleDbCommand cmdExcel = new OleDbCommand();
OleDbDataAdapter oda = new OleDbDataAdapter();
DataTable dt = new DataTable();
cmdExcel.Connection = connExcel;

connExcel.Open();
DataTable dtExcelSchema;
dtExcelSchema = connExcel.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, null);
string SheetName = dtExcelSchema.Rows[0]["TABLE_NAME"].ToString();
connExcel.Close();

connExcel.Open();
cmdExcel.CommandText = "SELECT * From [" + SheetName + "]";
oda.SelectCommand = cmdExcel;
oda.Fill(dt);
connExcel.Close();
return dt;