I finally got it. The description ALT/OPT means hold them down together and not like I thought hold ALT or OPT down and click (which I both tried).
If I hold down both ALT+OPT and click the preview button next to the make album button, the preview opens in the browser. So it does work on my (X)Ubuntu Linux.

For me, the case is solved. Thanks for the discussion.

Actually the ALT/OPT is intended to mean on Windows use the ALT key, on Macs use the OPT key. I guess you have a third party keyboard that has both keys, like a Logitech dual layout keyboard. The wiki pages are written primarily based on macOS with common Windows keyboard shortcuts like OPT and ALT. There are too many versions of Linux to cover and I don’t even have one machine, and no Linux user has ever chosen to edit or add to the wiki.