primary forgien key error

yes value exists.

I have tried the same by creating a POC with database and it worked for me without any problem. Kindly compare your code with the one below and see what you are missing. I feel somewhere the TownId field is getting incremented and that value is being picked while inserting into database.

aspx
   
   
   

aspx.cs
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings[2].ConnectionString);
            conn.Open();
            SqlCommand cmd = new SqlCommand("insert into Phase values(4,'Phase','" + TownName.SelectedValue + "')",conn);
           
            cmd.ExecuteNonQuery();
            conn.Close();
        }
    }

1.aspx


1.aspx.cs
con.Open();
cmd = new SqlCommand("insert into Phase values('"+PhaseID.Text+"','" + PhaseName.Text + "','" + TownName.SelectedValue + "')", con);
cmd.ExecuteNonQuery();

 If you want to store the TownName in Phase table then you're not using the foreign key references. 
So it is better to store town id in the phase instead of storing town name in phase table.
From the dropdownlist, you can get the SelectedItem value which will have the townid .Pass this townid to the phase table to store.

Guessing the logic I can guess wrong townid is being passed to insert phase query. The wrong townid is picked from the dropdownlist. This means the dropdownlist town id could be incremented by 1 so it could be like you are selecting the last item of the dropdown and inserting into phase table.

yes townId is present in the town table
i want to pass the townname to the database from dropdown(but remember the composite key is town id).

Does the TownID is present in the Town table?
Are you passing the TownID or the TownName (from Dropdown) to the database ?

if possible please post the code.