My first question would be why are you using --force-renew ? if you just do the “renew” then it will renew those certificates that need it, and not any others. That way it can be run on a daily basis.
Secondly, it looks like an older version of letsencrypt / certbot (version 0.4.1) I’m not sure what the latest version in the Ubuntu 16.06 package is, but might be worth checking you are uptodate. ( that worked on the same error - Fails when running cronjob )
On the error message itself, it’s almost certainly related to the PATH variable which is different in cron than when logged in as a user - see https://github.com/certbot/certbot/issues/1833