I am using LibreOffice 6.0.6.2 (french version on Ubuntu). I don’t see how to digitally sign a file. When I select digital signature, I don’t see any signature. When I try to save a file with the GPG button selected, the public keys can be seen, but when I chose one, I have a error message, the key is wrong, damaged or encrypting can’t be done.
Having the Same problem on English Ubuntu, I had a search for a solution and then realized my suite was installed through snap. I installed .deb packages from the libreoffice.org downloads page, and problem was solved. There’s a readme in the tar file that’s worth looking at for installation command.