validation error 'the travel request' is not available here

Content

I want to be about to have a rule:

the incident subject = the travel request event name + " - " + the travel requst event location,

but am getting a validation error 'the travel request' is not available here. You may need to use an entity function to refer to 'the travel request event name. (OPA-E00209)'.   

I have reviewed the documentation at http://documentation.custhelp.com/euf/assets/devdocs/november2016/PolicyAutomation/en/Default.htm#Guides/Policy_Modeling_User_Guide/Entities_and_relationships/Use_an_entity _or_relationship_in_a_rule.htm  but cannot seem to work it out.

'the incident subject' in the Global entity