Why won't my merged PDFs save in Preview on my Mac?

I followed these Apple instructions however there seems to be a step 5 missing on how to save and close the newly merged documents?

I open 1 PDF.

I then merge 60 PDFs into the side thumbnail column.

I then close the 1 PDF with the 59 other PDFs and the 1 PDF hasn't merged the 59 PDFs.

So I repeat and select Save at the end but same challenge.

So I repeat and select Export at the end but same challenge.

I'm using macOS Sierra Version 10.12.6 and Preview Version 9.0 (909.18).

Also, it's a Guest account and stuck in the German language as the Admin password is unknown.

Any suggestions please?

[Re-Titled by Moderator]