Oh. I think I see something that may be problematic. Try changing m="objects.terrain" to m="interactions.base.super_interaction"
Then try with or without the posture constraint again. You might also try removing _provided_posture_type, but with the constraint. I've read the tdesc before, but I'm not totally sure how _provided_posture_type functions in practice.
Edit: Also, unless you're wanting a flush animation to happen in some way, you should be able to safely get rid of the pre-add and post-run commodities. All they do is push the sim to move into a flush interaction after peeing, but since it's not an actual toilet, you shouldn't need that.
Also also, if you get stumped, my recommendation would be to look for a Maxis-made interaction that is as similar as possible to what you're attempting to do and just use that as a base, with the peeing animation and posture inserted into it... see if it makes any difference. I don't know how helpful that is, but it's what I usually do if I'm struggling. Off-hand, I'm not sure what all there is in the way of interactions that are ground-targeted that aren't just a movement-based thing. But I guess there's the list of super_affordances in terrain object, so you could look there for examples.
