How do I get rid of this error and successfully save the details in my databse

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ASP.NET

Line 18: string checkuser = "select(*)from Table where [UserName] = '"+TextBoxUsername.Text+ "'";
Line 19: SqlCommand com = new SqlCommand(checkuser, conn);
Line 20: int temp = Convert.ToInt32(com.ExecuteScalar().ToString());
Line 21: if(temp == 1)
Line 22: {

Hello folks, I have encountered this error which says "An exception of type 'System.Data.SqlClient.SqlException' occurred in System.Data.dll but was not handled in user code

Additional information: Incorrect syntax near '*'. " and its pointing towards Line 20.

I'm a beginner ,trying to learn Asp.net. Please help me out if anyone has a solution.
P.S : i also saw the other guy has same error, and i tried to change the code,but still have the same problem. Thanks in advance

What I have tried:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;

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

if(IsPostBack){
SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["RegistrationConnectionString"].ConnectionString);
conn.Open();
string checkuser = "select(*)from Table where [UserName] = '"+TextBoxUsername.Text+ "'";
SqlCommand com = new SqlCommand(checkuser, conn);
int temp = Convert.ToInt32(com.ExecuteScalar().ToString());
if(temp == 1)
{
Response.Write("User already exists");
}

conn.Close();
}
}
protected void Button1_Click(object sender, EventArgs e)
{
try{


SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["RegistrationConnectionString"].ConnectionString);
conn.Open();
string insertQuery = "insert into Table (UserName,Email,Password,Country) values (@Uname,@email,@password,@country)";
SqlCommand com = new SqlCommand(insertQuery, conn);
com.Parameters.AddWithValue("@Uname", TextBoxUsername.Text);
com.Parameters.AddWithValue("@email", TextBoxEmailid.Text);
com.Parameters.AddWithValue("@password", TextBoxpassword.Text);
com.Parameters.AddWithValue("@country", DropDownListCountries.SelectedItem.ToString());
com.ExecuteNonQuery();
Response.Redirect("Manager.aspx");
Response.Write("Registration is successful");

conn.Close();
}
catch(Exception ex){
Response.Write("Error :" +ex.ToString());
}
}
}