Insert Sequential Number into column C#

Question

I have this table called Articlebarcode:

In the column Code I need to insert a sequential number, that is, I have to read the maximum number that is there and insert the next one that is available ! I have to use the following line of code to see what is the available value:

SELECT ISNULL(MAX(Code), 0) + 1 FROM ArticleBarCode

Can anyone help me with this situation? Thank you !

This is the code I use to enter values in this table:

conn.Open();
comm.CommandText = @"INSERT INTO ArticleBarCode(Code, Code_Article, BarCode, CreatedBy, CreatedOn, ModifiedBy, ModifiedOn, IsDeleted)
                                   VALUES (@code, @codearticle, @barcode, 1, @date, 1, @date, 0)";
comm.Parameters.AddWithValue("@code", next);
comm.Parameters.AddWithValue("@codearticle", code);
comm.Parameters.AddWithValue("@barcode", numbercode);
comm.Parameters.AddWithValue("@date", DateTime.Now);
comm.ExecuteNonQuery();
conn.Close();

That next has to be the sequential value ! Everything else works.

I have this code that can fetch the next available value and then I try to insert:

 conn.Open();
 SqlCommand cmd5 = new SqlCommand("SELECT ISNULL(MAX(Code), 0) + 1 FROM ArticleBarCode", conn);
 Int32 next = (Int32) cmd5.ExecuteScalar();
 conn.Close();

 conn.Open();
 comm.CommandText = @"INSERT INTO ArticleBarCode(Code, Code_Article, BarCode, CreatedBy, CreatedOn, ModifiedBy, ModifiedOn, IsDeleted)
                    VALUES (@code, @codearticle, @barcode, 1, @date, 1, @date, 0)";
  comm.Parameters.AddWithValue("@code", next);
  comm.Parameters.AddWithValue("@codearticle", code);
  comm.Parameters.AddWithValue("@barcode", numbercode);
  comm.Parameters.AddWithValue("@date", DateTime.Now);
  comm.ExecuteNonQuery();
  conn.Close();

But when I do this, it gives me this mistake:

The INSERT statement conflicted with the FOREIGN KEY Constraint "Fk_articlebarcode_article". The Conflict occurred in database "Cardozugestdb", table "dbo. Article", column 'Code'.