Why does using ColumnText result in "The document has no pages" exception?

Document document = new Document(PageSize.A4.rotate());
ByteArrayOutputStream baos = new ByteArrayOutputStream();
PdfWriter writer = PdfWriter.getInstance(document, baos);
document.open();
ColumnText column = new ColumnText(writer.getDirectContent());
column.setSimpleColumn(new Phrase("text is very long ..."), 10, 10, 20, 20, 18, Element.ALIGN_CENTER);
column.go();
document.close();

I see that you have the following line:

column.go();

You did not use something like this:

int status = column.go();

If you did, and if you examined status, you would have noticed that the column object still contained some text.

What text? All the text.

There is a serious error in this line:

column.setSimpleColumn(new Phrase("text is very long ..."), 10, 10, 20, 20, 18, Element.ALIGN_CENTER);

You are trying to add the text "text is very long ..." into a rectangle with the following coordinates:

float llx = 10;
float lly = 10;
float urx = 20;
float ury = 20;

You didn't define a font, so the font is Helvetica with font size 12pt and you defined a leading of 18pt.

This means that you are trying to fit text that is 12pt heigh with an extra 6pt for the leading into a square that measures 10 by 10 pt. Surely you understand that this can't work!

As a result, nothing is added to the PDF and rather than showing an empty page, iText throws an exception saying: there are no pages! You didn't add any content to the document!

You can fix this, for instance by changing the incorrect line into something like this:

column.setSimpleColumn(new Phrase("text is very long ..."), 36, 36, 559, 806, 18, Element.ALIGN_CENTER);

An alternative would be:

column.setSimpleColumn(rect);
column.addElement(paragraph);

In these two lines rect is a Rectangle object. The leading and the alignment are to be defined at the level of the Paragraph object (in this case, you don't use a Phrase).