Having followed the Certbot Instructions in the (excellent) docs, I only have one question:
(my HTTP website is running nging with pip).
After the step No8 (Set up automatic renewal) I get no error, but I don't see any new entry in my crontab.
Is this normal?
As for now, everything is ok.
Also: sudo certbot renew --dry-run --force-renewal ends successfully
Thanks in advance
Please don't use this option, it's almost never useful or necessary. In the case of a --dry-run, it's not necessary at all. Please forget that option exists at all.
With regard to the cronjob: I don't know, the Certbot website has multiple instructions, depending on your specific situation. So you'd need to inform us how you installed Certbot. When using snap, it doesn't use a cronjob, but a systemd timer instead.
what about systemctl list-timers?
Do you mean this step 8?
echo "0 0,12 * * * root /opt/certbot/bin/python -c 'import random; import time; time.sleep(random.random() * 3600)' && sudo certbot renew -q" | sudo tee -a /etc/crontab > /dev/null
From: Certbot Instructions
Because that is a routine Linux command which should create a file. Did you enter it just as described?
Yes, exactly this step (echo ... /null).
I followed the instructions for nging and pip
Below are the timers and no (new) entry in my crontab. 
NEXT LEFT LAST PASSED UNIT ACTIVATES
Tue 2025-04-29 20:08:06 UTC 11h left Mon 2025-04-28 20:08:06 UTC 12h ago systemd-tmpfiles-clean.timer systemd-tmpfiles-clean.service
Tue 2025-04-29 22:26:44 UTC 13h left Tue 2025-04-29 07:28:40 UTC 1h 4min ago apt-daily.timer apt-daily.service
Wed 2025-04-30 00:00:00 UTC 15h left Tue 2025-04-29 00:00:01 UTC 8h ago dpkg-db-backup.timer dpkg-db-backup.service
Wed 2025-04-30 00:00:00 UTC 15h left Tue 2025-04-29 00:00:01 UTC 8h ago exim4-base.timer exim4-base.service
Wed 2025-04-30 03:14:54 UTC 18h left Tue 2025-04-29 01:48:08 UTC 6h ago man-db.timer man-db.service
Wed 2025-04-30 06:26:30 UTC 21h left Tue 2025-04-29 06:56:13 UTC 1h 36min ago apt-daily-upgrade.timer apt-daily-upgrade.service
Sun 2025-05-04 03:10:50 UTC 4 days left Mon 2025-04-28 11:53:40 UTC 20h ago e2scrub_all.timer e2scrub_all.service
Mon 2025-05-05 01:34:28 UTC 5 days left Mon 2025-04-28 11:53:40 UTC 20h ago fstrim.timer fstrim.service
- - - - logrotate.timer
9 timers listed.
Yes, I copied/pasted, as all previous ones.
My machine is a clean Debian 12.10
With the pip instructions, the crontab step is indeed manual (compared with the automatically installed systemd timer with snap).
Was the file /etc/crontab actually created when you ran the "Set up automatic renewal" step?
Yes, I see a /etc/crontab file and the last line is the:
0 0,12 * * * root /opt/certbot/bin/python -c 'import random; import time; time.sleep(random.random() * 3600)' && sudo certbot renew -q
So, I assume that the answer to my question is:
Don't look only the user's crontab (crontab -l) but also the system wide /etc/crontab file
Thanks for your help 
What command did you use to verify that?
What does this say
sudo crontab -l
You should also be able to look at the logs (I think in /var/log/letsencrypt) and confirm that it's actually running each day and confirming that there aren't any certificates that need renewal yet.