How to INSERT text into database table using VB.NET ?

This has been already partially answered, but I'd like to add a few points:

- As already said, you don't need to use ToString for a literal

... ,"NOT AVAILABLE")'

should be sufficient, although this could also be added as a default in the database if the value is always the same in the insert.

- More important than that is that never ever concatenate the input from the user as literal to the SQL statement. The only correct way is to use parameters in order to be safe from SQL injection and help with type conversions etc.

- Third thing is the usage of dispose or using. You should always dispose the disposable objects. I feel that the easiest way is to use using statements instead of calling the Dispose method. For example in your code you do not dispose the SqlCommand.

- Dates, never store the dates in any other format than in a date column. When using proper data types in the database it's easy to query, modify, calculate with the data. So instead of converting the date into a string, use a parameter and store DateTime.Now into it as-is.

- Dept_Id, not sure if this is the key and how it's supposed to be handled, but if this column is intended to be the actual primary key for the table, you probably should let the database to decide the value for it in order to prevent duplicates. If needed, have a look at IDENTITY columns for example

- The insert statement is executed by calling ExecuteReader. However, as this is an insert statement, nothing is expected to be returned from the database. A commonn practice is to use ExecuteNonQuery

If you have time, I'd suggest going through Properly executing database operations[^] . It explains the fundamentals for executing commands from different point of views.

I re-coded the snippet you gave and formatted it a little better as well as removing some redundancies.

Here is how it would look without code included:

INSERT INTO department(dept_id, dept_name, dept_created, dept_modified) 
  VALUES ( "1", "Department Name", "2018-2-15", '"NOT AVAILABLE',ToString")

As you can tell, the "NOT AVAILABLE" would not be shown as a string (within Visual Studio) which leads to the errors I presume you have seen but not posted. By using Strings.Format method, as you can tell, the query becomes much more readable and easy to edit or updated. I also used the an object initializer (named With) when creating the SqlConnection. The code provided below should fix any problems you've had. Hope I helped!

Dim conn As New SqlConnection() With { .ConnectionString = "<string>" }
Dim reader As SqlDataReader
Try
    conn.Open()
    Dim strDate As String = DateTime.Now.ToString("yyyy-MM-dd")
    Dim query As String = "INSERT INTO department(dept_id, dept_name, dept_created, dept_modified) VALUES ("
    query = query & String.Format("{0}, {1}, {2}, 'NOT MODIFIED')",
                                  D_ID.box.Text, D_NAME_BOX.Text, strDate)
    Dim command As new SqlCommand(query, conn)
    command.ExecuteReader()
    load_dgv()
    conn.Close()
Catch ex As Exception
    MessageBox.Show(ex.Message)
Finally
    TextBox1.Clear() : TextBox2.Clear() : TextBo3.Clear()
    ComboBox1.Text = String.Empty
    conn.Dispose()
    MessageBox.Show("Department added successfully.")
End Try

Const query As String = "INSERT INTO department(dept_id, dept_name, dept_created, dept_modified) VALUES (@dept_id, @dept_name, @dept_created, 'NOT MODIFIED')"

Using conn As SqlConnection = New SqlConnection("...")
    Using command As SqlCommand = New SqlCommand(query, conn)
        command.Parameters.AddWithValue("@dept_id", D_ID.box.Text)
        command.Parameters.AddWithValue("@dept_name", D_NAME_BOX.Text)
        command.Parameters.AddWithValue("@dept_created", strDate) ' NB: Don't store dates as strings!
        
        conn.Open()
        command.ExecuteNonQuery() ' NB: Don't use ExecuteReader when the query doesn't return anything!
    End Using
End Using