Altium error: Open emitter pin and power pin object!
As Dave says, not everything the DRC flags is an actual error. It's up to you to understand your design and decide what's really an error.

What I'll add is the way to fix it: 

Make a copy of the optocoupler part in your own library, and change the pin properties to reflect how you'll use the part. To minimize DRC checking, you can set the pin type to "passive". Or for this circuit arrangement you can mark it as a power pin.

Now use your new part in your design instead of the one provided by Altium. Altium can't predict how every customer wants to use each part, so their library creation decisions might not be right for you, and you should be prepared to make your own library components when needed.