Incorrect date value: '17/4/2020 2:38:44 pm' for column 'stock_sales_system'.'customer'.'startdate' at row 1

using (SqlCommand cmd = new SqlCommand(connection))
{
   cmd.CommandText = "INSERT INTO stock_sales_system.customer (name, address, city, telephone, datefield, description, unknownfield1, unknonwnfield2) VALUES (@name, @address, @city, @telephone, @date, @description, @field1, @field2)";
   cmd.Parameters.Add("@name", SqlDbType.NVarChar).Value = txtCustomerName.Text;
   cmd.Parameters.Add("@address", SqlDbType.NVarChar).Value = txtAddress.Text;
   cmd.Parameters.Add("@city", SqlDbType.NVarChar).Value = cbxCity.Text;
   cmd.Parameters.Add("@telephone", SqlDbType.NVarChar).Value = txtTelephone.Text;
   cmd.Parameters.Add("@date", SqlDbType.DateTime).Value = dtpDate.Value;
   cmd.Parameters.Add("@description", SqlDbType.NVarChar).Value = txtDescription.Text;
   cmd.Parameters.Add("@field1", SqlDbType.Int).Value = 0;
   cmd.Parameters.Add("@field2", SqlDbType.Int).Value = 0;
   int result = cmd.ExecuteNonQuery();
}

Here you will have to provide proper column names as I had to guess them since you did not show them.

The bad habits you should lose asap:

• building SQL queries by concatenating strings obtained from use inputs; this is a no-go as this leaves your code wide open to SQL injection attacks.
• not providing column names in your SQL queries.
• treating everything as strings. This is what you do when you enclose every value between single quotes. Please always use proper types, and make sure to follow proper syntax for each of them. Using query parameters discharge you from putting any quote in the query, anyway.