1. install snapd on Debian

 

$ sudo apt update
$ sudo apt install snapd

$ snap install core
core 16-2.45.2 from Canonical✓ installed

$sudo snap refresh core
2. Install the Certbot
sudo snap install --classic certbot

Install the Cert for nginx

sudo certbot --nginx

It will create a timer file with crontab /etc/cron.d/certbot

# /etc/cron.d/certbot: crontab entries for the certbot package
#
# Upstream recommends attempting renewal twice a day
#
# Eventually, this will be an opportunity to validate certificates
# haven't been revoked, etc. Renewal will only occur if expiration
# is within 30 days.
#
# Important Note! This cronjob will NOT be executed if you are
# running systemd as your init system. If you are running systemd,
# the cronjob.timer function takes precedence over this cronjob. For
# more details, see the systemd.timer manpage, or use systemctl show
# certbot.timer.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

0 */12 * * * root test -x /usr/bin/certbot -a \! -d /run/systemd/system && perl -e 'sleep int(rand(43200))' && certbot -q renew

Verify it

systemctl list-timers
3. Install the Cert

sudo certbot –nginx

Then select the domain, and action:

Which names would you like to activate HTTPS for?
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
1: your.domain.name
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Select the appropriate numbers separated by commas and/or spaces, or leave input
blank to select all options shown (Enter 'c' to cancel): 1
Cert not yet due for renewal

You have an existing certificate that has exactly the same domains or certificate name you requested and isn't close to expiry.
(ref: /etc/letsencrypt/renewal/your.domain.name.pem)

What would you like to do?
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
1: Attempt to reinstall this existing certificate
2: Renew & replace the cert (limit ~5 per 7 days)

Select the 1. Then you will see below:

Congratulations! Your certificate and chain have been saved at:
/etc/letsencrypt/live/your.domain.name/fullchain.pem
Your key file has been saved at:
/etc/letsencrypt/live/your.domain.name/privkey.pem
Your cert will expire on 2021-01-24. To obtain a new or tweaked
version of this certificate in the future, simply run certbot again
with the "certonly" option. To non-interactively renew *all* of
your certificates, run "certbot renew"