Preview says "The file couldn't be saved" for a pdf document Note this is different from the similar question Preview fails to save pdf forms because that other question is about pdf *forms but this is one is just a normal pdf.

I am using Preview for the first time on Catalina - but am a veteran on earlier platforms. I am unable to save an updated PDF

 

This is a PDF that I added some textboxes and a Tools|Annotations|Signature.  

Any ideas?
Once you annotate a regular PDF doc, Preview will prevent you from directly saving the file.

You will have to remove the annotations or, as you already found, do a Print -> Save to PDF.