Even if you do solve it, that is a dangerous way to do it.
What happens if a different user calls that same code at the same time? Or even after you do this, but before you add your record to the database? Answer: you both end up with the same number.
First off: ditch the conversions, and return it via a scalar:
SqlCommand com = new SqlCommand("select max(makeid) from make",con);
int a = com.ExecuteScalar();
txtmakeid.Text = Convert.ToString(a);
Which would cure your immediate problem.
But it won't solve the duplicate values!