The "Check Availability" username button still recognized as exists, while the database the username is not exists

The "Check Availability" username button still recognized as exists, while
the database the username is not exists

How do i prevent a button to continue when there are an errors appears?
I already can check the availability of username in database, but even
though the username is not exists in the database, the "Check
Availability" button still recognized it as exists.
Here is the screenshots:
Image above show the username "Seranne" already exists, but in the
database, Seranne is not exists.

Here is the code:
else if (textBox1.Text.Length > 0 && textBox2.Text == textBox3.Text)
{
label5.Visible = false;
label7.Visible = false;
conn.Open();
CheckUsername();
if (CheckUsername() == false)
{
return;
}
cmd.CommandText = "INSERT INTO [Member] ([Username],
[Password], [UserType]) VALUES (@Username, @Password,
@UserType)";
cmd.Parameters.Add("Username",
System.Data.OleDb.OleDbType.VarChar);
cmd.Parameters["Username"].Value = this.textBox1.Text;
cmd.Parameters.Add("Password",
System.Data.OleDb.OleDbType.VarChar);
cmd.Parameters["Password"].Value = this.textBox2.Text;
cmd.Parameters.Add("UserType",
System.Data.OleDb.OleDbType.VarChar);
cmd.Parameters["UserType"].Value = this.comboBox1.Text;
int numberOfRows = cmd.ExecuteNonQuery();
conn.Close();
System.Media.SoundPlayer sound = new
System.Media.SoundPlayer(@"C:\Windows\Media\Windows
Notify.wav");
sound.Play();
var dialogresult = MessageBox.Show("Your username and
password has been recorded", "Congratulations",
MessageBoxButtons.OK);
CreateTable();
if (dialogresult == DialogResult.OK)
{
this.Hide();
Form1 form = new Form1();
form.ShowDialog();
this.Close();
}
private void CheckUsername()
{
OleDbConnection conn = new OleDbConnection();
conn.ConnectionString =
@"Provider=Microsoft.ACE.OLEDB.12.0;Data
Source=\Archives\Projects\Program\Sell System\Sell
System\App_Data\db1.accdb";
conn.Open();
OleDbCommand cmd = new OleDbCommand("SELECT [Username] FROM
[Member]", conn);
cmd.Parameters.Add("Username",
System.Data.OleDb.OleDbType.VarChar);
cmd.Parameters["Username"].Value = this.textBox1.Text;
OleDbDataReader reader = cmd.ExecuteReader();
if (reader.HasRows)
{
System.Media.SoundPlayer sound = new
System.Media.SoundPlayer(@"C:\Windows\Media\Windows
Notify.wav");
sound.Play();
MessageBox.Show("Username already exists! Please use
another username", "Warning");
return false;
}
else
{
System.Media.SoundPlayer sound = new
System.Media.SoundPlayer(@"C:\Windows\Media\Windows
Notify.wav");
sound.Play();
MessageBox.Show("Username is not exists!",
"Congratulations");
return true;
}
}
edit: even though the username is not exists in the database, "Check
Availability" button still recognized it as exists and this is the reason
that i can't proceed.
How do i solve this? Thanks in advance!