E: Sub-process /usr/bin/dpkg returned an error code (1) - but how do I find the meaningful error messages in APT's output?

Apt logs it's actions sequentially, just like a human does.

Simply read the output line-by-line.

Here is a non-apt example. Read it line-by-line and you will see that it tells a story:

I am going to build a tower out of five blocks...
Clearing a working surface in the kitchen.
Kitchen: Put 7 dirty dishes from the counter into the dishwasher
Kitchen: Wiped the counter clean
Opening the box of blocks.
Warning: There are only three blocks in the box.
Build: Placed the first block.
Build: Placed the second block.
Build: Placed the third block.
Build: ERROR: Cannot keep building - ran out of blocks.
Closing the box of blocks.
ERROR (summary): Failed to complete the five-block tower.

Apt and dpkg logging works in exactly the same way - you read the story line-by-line. You see the package manager embark upon it's adventure: prepare, run subtasks, encounter non-fatal problems (Warnings), overcome adversity, etc.

Most of the output is routine, but you need it to mark apt's progress. That context is how you understand what was happening. It really is a story.

Error (1) is common. It's a summary error code, indicating that the problem occurred to a subtask ("some problem building"). Jump backward in the story to where that particular subtask occurred, and you will see the specific detail ("ran out of blocks").

In the complete output of sudo apt update or sudo apt upgrade or sudo apt install -f you should find some messages from dpkg about what actually went wrong.

These lines will start with dpkg: since that's the name of the program returning the error. The lines of output immediately preceding or following these lines are often the most helpful.

A few examples of the many possible errors you might see:

Setting up install-info (6.4.90.dfsg.1-1build1) ...
/usr/sbin/update-info-dir: 3: /etc/environment: $: not found
dpkg: error processing package install-info (--configure):
    subprocess installed post-installation script returned error exit status 127

This means that the post-installation script could not run. Shells use an exit status of 127 to mean "command not found," so a command called by the post-inst script was not found. The lines before the dpkg line give a hint as to the reason: there is something wrong with the /etc/environment file (which should set environment variables like PATH).

start: Unable to connect to Upstart: Failed to connect to socket /com/ubuntu/upstart: Connection refused
No apport report written because the error message indicates its a followup error from a previous failure.
dpkg: error processing package runit (--configure):
 subprocess installed post-installation script returned error exit status 1
dpkg: dependency problems prevent configuration of git-daemon-run:
 git-daemon-run depends on runit; however:
  Package runit is not configured yet.

The above error was caused by a bug in the runit package, which expected to find Upstart installed, although it had been succeeded by systemd as the default init system for Ubuntu. So, the message failed to connect to Upstart is the best hint, but we need the context to find out how this is causing the package management problem.

Unpacking libjline-java (from .../libjline-java_1.0-1_all.deb) ...
dpkg: error processing /var/cache/apt/archives/libjline-java_1.0-1_all.deb (--unpack):
 trying to overwrite '/usr/share/java/jline.jar', which is also in package scala 2.9.2-400

This means there are package conflicts, perhaps caused by having a mixture of repository versions or third party repositories.

TL;DR

In any case where you see the error Sub-process /usr/bin/dpkg returned an error code (1), you need to look above for lines starting with dpkg: and the lines before and after them for useful clues to what went wrong. Try searching for these specific errors.

If you are asking a question here or on another support site, make sure you include the command you ran and the complete output, not just the summary error messages.