Вы пытаетесь создать несколько таблиц с одинаковым именем в al oop. Я полагаю, ваш код должен выглядеть примерно так:
// StringBuilder is better way to creating a string in a loop,
// because it doesn't allocate new string on each concatenation
StringBuilder command = new StringBuilder("create table ");
command.Append(textBox3.Text).Append("(");
string separator = "";
// It is better to give more descriptive names to variables
foreach (string columnName in listBox1.Items)
{
// You forgot to specify column type
command.Append(separator)
.Append(columnName)
.Append(" varchar(1000)");
separator = ",";
}
command.Append(")");
// SqlCommand and SqlConnection implement IDisposable,
// so it is better to wrap their instantiation by 'using' statement
// in order to free corresponding resources
using (SqlCommand sqlCommand = new SqlCommand(command.ToString(), SC1)) {
sqlCommand.ExecuteNonQuery();
}