Hi Alento,
Thanks for getting back and I do apologise for the lack info.
If someone is looking at this, in fact the issue came up before and here is the answer:
By running this line we succeeded in installing LetsEncrypt:
sudo certbot --authenticator standalone --installer apache -d thedomain.com --pre-hook "systemctl stop apache2" --post-hook "systemctl start apache2"
replace "thedomain.com" as appropriate.