In Pages, how do I convert a flowable formal into fixed format?

If you are referring to an already exported EPUB where you chose reflowable over fixed, then export the document to EPUB again and choose Fixed.

If instead, you were using non-standard terms for word processing layout document as flowable and fixed format for Layout format documents, then you cannot convert the word processing document to page layout without wiping its contents.

That would produce this dialog:

and it means what it says. If you start with a page layout document, then you can convert it to a word processing document without loss of content.

The last time I looked, the Kindle book generation software wanted MS Word document source. I know that at one time, it also accepted Pages documents but that is likely now aging documentation that no longer applies. When Apple generates that EPUB from the Pages export, it is intended to be used in Apple Books and does incorporate specific CSS styling for that end.

In the Finder, you can click on the original file and press cmd+D to duplicate it. That will create filename copy.pages. Also, in the Finder, you can right-click on that new filename and select rename to do just that.

Open the newly renamed Pages document that you want to export as a Fixed layout EPUB. Before you do that, visit Pages File menu > Page Setup… and click on that Paper Size selector. Choose Manage Custom Sizes… and then on that new panel, click the + symbol. That will create a new custom page name untitled. Click that and change its name to 6 x 9. Then set the width and height accordingly. If you are OK with the default margin settings, click OK and your Pages document will resize to 6 x 9. Note here, that a Fixed Layout EPUB does not support fully-justified text and the export process will also tell you that.

Now, export to EPUB as Fixed Layout and any other settings on the EPUB panel, then click Save… Apple Books will open the EPUB as a 6 x 9 document.

Why not export the document to EPUB as a fixed layout in that panel and then open that EPUB in Apple Books and print from there?

You are welcome.