RN10400 Nextcloud & Certbot-auto

Hi, I have a ReadyNAS RN10400 with Firmware 6.10.2 on which I have installed the following apps from RNXtras:

MySQLDB, Nextcloud and PHP 7.2. Nextcloud is working fine, but before using it in anger, I am trying to use LetsEncrypt certificates to enure secure the installation. I have downloaded the version of "certbot-auto" corresponding to Debian "Jessie", but whenever I run it, regardless of what options are used, I get the following error:

"The following packages have unmet dependencies:
python-virtualenv : Depends: python-pip-whl (>= 8.1.1-2) but 1.5.6-5 is to be installed
virtualenv : Depends: python3-virtualenv but it is not going to be installed
E: Unable to correct problems, you have held broken packages."

The current Python level installed is 2.7.

I note from other community posts that other users have been able to use Certbot-auto successfully, but I'm guessing that these are from prior to the update to Debian Jessie.

Please advise how I can resolve this issue.