How to save data in ms access using C#

command.CommandText = "insert into tblcustomer where customername='" + txt_customername.Text + "' and Name='" + txt_Name.Text + "' and Surname='" + txt_Surname.Text + "'";

Not a solution to your question, but another problem you have.
Never build an SQL query by concatenating strings. Sooner or later, you will do it with user inputs, and this opens door to a vulnerability named "SQL injection", it is dangerous for your database and error prone.
A single quote in a name and your program crash. If a user input a name like "Brian O'Conner" can crash your app, it is an SQL injection vulnerability, and the crash is the least of the problems, a malicious user input and it is promoted to SQL commands with all credentials.
SQL injection - Wikipedia[^]
SQL Injection[^]
SQL Injection Attacks by Example[^]
PHP: SQL Injection - Manual[^]
SQL Injection Prevention Cheat Sheet - OWASP[^]

First of all, all notes about SqlInjection are very important! So, please, read about it before you start doing anything.

Instead of such of statement:

command.CommandText = "insert into tblcustomer where customername='" + txt_customername.Text + "' and Name='" + txt_Name.Text + "' and Surname='" + txt_Surname.Text + "'";

you should use parameterized query:

command.CommandText = "INSERT INTO tblcustomer (customername, Name, Surname)
VALUES(@custname, @name, @surname)";
command.Parameters.AddWithValue("@custname", txt_customername.Text);
command.Parameters.AddWithValue("@name", txt_Name.Text);
command.Parameters.AddWithValue("@surname", txt_Surname.Text);

For further details, please see:
OleDbParameterCollection.AddWithValue Method (String, Object) (System.Data.OleDb)[^]

using (OleDbConnection connection = new OleDbConnection(sConStr))
{
    connection.Open();
    //here the code responsible for command creation and execution
}<pre>

private void btn_add_Click(object sender, EventArgs e)
{

string sqlQuery = "INSERT INTO tblinventory(`product_code`,`product_name`,`price`,`category`,`stock`) values (?,?,?,?,?)";
using (OleDbConnection conn = new OleDbConnection(@"Provider=Microsoft.ACE.Oledb.12.0;Data Source=db.accdb"))
using (OleDbCommand cmd = new OleDbCommand(sqlQuery, conn))
{
if ((txtproductcode.Text == "") || (txtproductname.Text == "") || (txtprice.Text == "") || (txtcategory.Text == "") || (txtquantity.Text == ""))
{
MessageBox.Show("Please fill the all fields!");
}
else {
conn.Open();
cmd.Parameters.AddWithValue("@product_code", txtproductcode.Text);
cmd.Parameters.AddWithValue("@product_name", txtproductname.Text);
cmd.Parameters.AddWithValue("@price", txtprice.Text);
cmd.Parameters.AddWithValue("@category", txtcategory.Text);
cmd.Parameters.AddWithValue("@stock", txtquantity.Text);
MessageBox.Show("Successfully Added!");

txtproductcode.Clear();
txtproductname.Clear();
txtprice.Clear();
txtquantity.Clear();
txtcategory.ResetText();
cmd.ExecuteNonQuery();

}
}
}