I get "undefined" as a returned value for {{{encodeURI lastExportDateTime}}}, and no value at all for {{{lastExportDateTime}}} (that's probably the reason why the encodeURI function returns undefined). What can be the reason for this? I know that the flow has run before, so there should be a lastExportDateTime in the system.
If your export is set to delta, then integrator.io automatically creates a field in your export data named "lastExportDateTime". Verify that your export is set to Delta. When you return to the Relative URI editor, you should see the field listed in the Resources available for your handlebars template section, if that still doesn't work for you, I would suggest you visit an office hours session or open a support ticket to identify the root cause of your specific situation.