How do I stop it from printing in landscape mode instead of portrait mode?

It is an issue with LibreOffice. Go to Format>Page and change the Format from Letter to A4

Go back to your print screen and all should be right in the world.

I donā€™t have enuf points to upvote (bogus), but wanted to thank you for your answer. Itā€™s amazing that no matter how high-tech we get, itā€™s always printing that drags us back to stone knives and bearskins mode.

Thank you for your advise! Changing the page format from Letter to A4 solved my printing problem. Now I can print in portrait mode instead of landscape.

Add my thanks; I was seeing the problem on all documents. It would even do it saving as PDF. Selecting A4 format, clicking apply, then selecting back to ā€œLetterā€ (so my printer wouldnā€™t choke) fixed the problem.

Same problem with LO 5.4.1. I downloaded Version 5.3.4.2 and it prints fine.

Problem continues with LO Version: 5.3.6.1. Switching to A4 worked.

Suppose you were on ā€˜Letterā€™ (8.5 inch width by 11.0 inch height; 215.9 by 279.4 mm).
A workaround I would suggest but cannot test myself is to omit the predefined setting for width and height and to replace them by slightly reduced values. Reduce each by a 1/100 of an ā€œinchā€, and the printing to vertical pages should work.

The far better variant is, of course, to shift to the internationally standardized A4 if you have a chance to pull it through.

Please regard the recent amandments to my previous answer.

Thanks for this workaround. One problem I see is that once the bug particular to this problem is fixed, individuals might have to reset their templates back to the original dimensions.

Another workaround is to bring up the printer page, then click on ā€œmore.ā€ click on Brochure. click on 2 sides, click Brochure off. The format has been corrected and you can print in Portrait. Unfortunately you have to do this every time you print anything.

MacBookPro Mid 2015, macOS 10.13.3, LibreOffice v5.4.5.1.

Opening a few existing Writer documents and a newly created one shows portrait as expected, so Iā€™m not having a problem there.

Landscape issue still exists when opening existing spreadsheets, but newly created/saved spreadsheets are showing portrait properly. Tested using both .xlsx and .ods formats.

With this in mind, I ā€˜Saved Asā€¦ā€™ a couple spreadsheets to see what would happen. Saving as .xlsx shows portrait, saving as .ods shows landscape, whether in the same or a different directory. Based on that, Iā€™d say the issue now lies with existing .ods files created using version 5.2.7 (most likely). To support this claim, I copied & pasted the entire contents of an existing .ods into a newly created .odsā€¦portrait!

In all my testing, I completely ā€˜Quitā€™ LibreOffice each time.

Hopefully this is a repeatable thing and the developers can find a fix! Iā€™d rather not have all my users copy+paste a bunch of files or even re-save them in another format.