Thanks bernie.
To your suggestion to try using the "Manual binary install" from
https://calibre-ebook.com/download_linux, I have done that numerous times with LM 19.1 Cinnamon on VB 6.0.4, and on at least two other distributions of Linux Mint 19.1 Mate and Xfce with 6.0.4. I then re-installed LM 19.1 Cinnamon and attempted a fresh installation of calibre with 6.0.6. The errors that I posted earlier are after the fresh installation of LM 19.1 Cinnamon and VB 6.0.6.
(In fact, I spent much of yesterday trying to get calibre to install on LM 19.1).
Here is the result of running calibre in debug mode:
stewart@stewart-LM191:~$ calibre-debug -g
calibre 3.40.1 embedded-python: True is64bit: True
Linux-4.15.0-47-generic-x86_64-with-debian-buster-sid Linux ('64bit', 'ELF')
('Linux', '4.15.0-47-generic', '#50-Ubuntu SMP Wed Mar 13 10:44:52 UTC 2019')
Python 2.7.15
Linux: ('debian', 'buster/sid', '')
Interface language: None
Traceback (most recent call last):
File "site.py", line 77, in main
File "site-packages/calibre/debug.py", line 268, in main
File "site-packages/calibre/gui_launch.py", line 73, in calibre
File "site-packages/calibre/gui2/__init__.py", line 13, in <module>
ImportError: cannot import name QDesktopServices
Segmentation fault (core dumped)
stewart@stewart-LM191:~$