Cannot use command at on Ubuntu 22.04

On a relatively new instance of Ubuntu 22.04, I installed at:

sudo apt update
sudo apt install at

Now when I try to use it as a general user, I am receiving error: "You do not have permission to use at" for a general user

I found this article, which is an exact match for the error message, but references /etc/at.allow, which does not exist. File /etc/at.deny does exist but does not have this user (dennis) named within it.

In debugging, I tried adding a file /etc/at.allow, with only the desired user name in its content, as follows, and the error changed:

> sudo su -
# echo 'dennis' > /etc/at.allow
# chmod a+r /etc/at.allow
# atd restart
# exit
> at now
warning: commands will be executed using /bin/sh
Cannot open lockfile /var/spool/cron/atjobs/.SEQ: Permission denied

I found an answer for this permissions problem on StackOverflow that said permissions of that .SEQ file should be 777. I doubt that is true at all (and is instead probably a brute-force way to get around some not-understood problem), but I did try it, with the following result:

> sudo chmod 777 /var/spool/cron/atjobs/.SEQ
> echo 'ls' | at now
warning: commands will be executed using /bin/sh
job 5 at Wed May 24 21:05:00 2023
Can't signal atd (permission denied)

What else might I try to get this to work? Or what logs should I provide?

This was the result of aptitude not doing its post-installation work ("processing triggers") due to some held packages (notably, handbrake-related stuff). Once I removed the 'held' (and not needed) packages, I removed at and reinstalled:

sudo apt purge at
sudo apt install at

This solved the problem.