Whether I use Save As or Export As, I get the following error message.
Info on LO version here:
Help, please!
From what I know, this problem on MacOS is difficult to fix.
Is your folder on the machine or a remote folder? You can try that one: I get "General input/output error while accessing" when trying to open a file on an NFS share. But I’m not that optimistic.
Does it happen for any document or only this one?
Give LibreOffice full disk access in System Preferences > Security & Privacy. This is a security feature of MacOS. Cheers Al
See this site on how to do it if you aren’t sure: Explained: What is Full Disk Access & Full Permissions on macOS Mojave .