Add check for defined

manifests/init.pp

@@ -360,10 +360,12 @@ class mastodon (
     timeout     => 0,
     require     => File["${mastodon_home}/live/.env.production"],
   }
+  if (!defined(Class['letsencrypt'])) {
     class { 'letsencrypt':
       email             => $cert_admin_mail,
       renew_cron_ensure => 'present',
     }
+  }
   letsencrypt::certonly { $hostname:
     domains            => [$hostname],
     pre_hook_commands  => ['systemctl stop apache2'],