How to resolve error 'Unresolved dependency System ...' encoutered when trying to upgrade Drupal core?
The reason for your error is because of the last line in the info file of the Relation module, i.e.:


  dependencies[] = system (>=7.33)


In regular English it is something like "This version of the Relation module requires Drupal core to be at a level which is at least 7.33".

To resolve it perform these steps:


Disable of the Relation module.
Retry your core upgrade again (that error should not show up anymore, as per step "1.").
Re-enable the Relation module again.


I'd be interested to hear what result you get in that scenario ...

Note: since your site is pre 7.32, you may want to also work on things such as what is in suggested in the answers to Drupal SA-CORE-2014-005 - How to tell if my server / sites were compromised?