what is the value cmd.ExecuteScalar()

The documentation[^] states it clearly:

Okay, just go like this :

var commandStr= "If not exists (select name from sysobjects where name = 'Customer') CREATE TABLE Customer(First_Name char(50),Last_Name char(50),Address char(50),City char(50),Country char(25),Birth_Date datetime)";

using (SqlCommand command = new SqlCommand(commandStr, con))
command.ExecuteNonQuery();

Use ExecuteNonQuery if no return value is required. Use ExecuteScalar to return just one value and capture it ( like a COUNT or a SUM )

ExecuteScalar - Executes a command that returns a single value.
ExecuteReader - Executes a command that returns a set of rows.
ExecuteNonQuery - Executes a command that updates the datebase or change the database structure. This method returns the number of rows affected.