Mov site-packages/enum folder to site-packages-2. Actually I'm not sure if enum 34 can be installed on pythonista... The usual fix for that particular error is to delete the enum folder, and force quit pythonista.
I don't think you are going to have success installing opcua on 2.7, as at least some of its dependencies have c code. You might have better luck on 3.6, but probably will need to do some manual dependency installs, as pip doesn't deal well with complicated requirements.