Comment 4 for bug 1794981

This looks odd, I checked tidy/libtidy if they would conflict but they work fine:

$ sudo apt install tidy
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
  grub-pc-bin
Use 'sudo apt autoremove' to remove it.
The following additional packages will be installed:
  libtidy5
The following NEW packages will be installed:
  libtidy5 tidy
0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 185 kB of archives.
After this operation, 881 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
Get:1 http://archive.ubuntu.com/ubuntu bionic/main amd64 libtidy5 amd64 1:5.2.0-2 [160 kB]
Get:2 http://archive.ubuntu.com/ubuntu bionic/universe amd64 tidy amd64 1:5.2.0-2 [25.5 kB]
Fetched 185 kB in 3s (59.3 kB/s)
Selecting previously unselected package libtidy5.
(Reading database ... 94444 files and directories currently installed.)
Preparing to unpack .../libtidy5_1%3a5.2.0-2_amd64.deb ...
Unpacking libtidy5 (1:5.2.0-2) ...
Selecting previously unselected package tidy.
Preparing to unpack .../tidy_1%3a5.2.0-2_amd64.deb ...
Unpacking tidy (1:5.2.0-2) ...
Setting up libtidy5 (1:5.2.0-2) ...
Setting up tidy (1:5.2.0-2) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...

It seems you installed 18.04 and there was no upgrade to tidy/libtidy since 18.04 was released so I have to wonder why it was updated now according to your logs.

This case is odd, but I can't see the root cause.
If you have any background on what triggered this upgrade and how you got the error I'd appreciate, but from the logs so far I can't take any action.

I checked the supposed owner of the file:
libtidy5: /usr/lib/libtidy.so.5

But then I realized the 5.4 in your bug report.

These are the official/supported versions of tidy in Ubuntu right now
 tidy | 20091223cvs-1ubuntu2 | precise | source, amd64, armel, armhf, i386, powerpc
 tidy | 20091223cvs-1ubuntu2.1 | precise-security | source, amd64, armel, armhf, i386, powerpc
 tidy | 20091223cvs-1ubuntu2.1 | precise-updates | source, amd64, armel, armhf, i386, powerpc
 tidy | 20091223cvs-1.2ubuntu1 | trusty | source, amd64, arm64, armhf, i386, powerpc, ppc64el
 tidy | 20091223cvs-1.2ubuntu1.1 | trusty-security | source, amd64, arm64, armhf, i386, powerpc, ppc64el
 tidy | 20091223cvs-1.2ubuntu1.1 | trusty-updates | source, amd64, arm64, armhf, i386, powerpc, ppc64el
 tidy | 20091223cvs-1.5 | xenial | source
 tidy | 20091223cvs-1.5 | xenial/universe | amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
 tidy | 1:5.2.0-2 | bionic/universe | amd64, arm64, armhf, i386, ppc64el, s390x
 tidy | 1:5.2.0-2 | cosmic/universe | amd64, arm64, armhf, i386, ppc64el, s390x
 tidy | 1:5.2.0-2 | disco/universe | amd64, arm64, armhf, i386, ppc64el, s390x
 tidy | 2:5.6.0-6 | disco-proposed/universe | amd64, arm64, armhf, i386, ppc64el, s390x

There is no 5.4 especially not for Bionic which you are on.
I assume you installed "tidy" from an external source and now something installs libtidy5 by dependency and both packages conflict.

Remove or fix the external package to get things right.