Hello :)
ATM it looks like Ash-Greninja is locked to the IVs it gets in game 20/31/20/31/20/31. IIRC it is IV locked, which is fine, but bottle caps can fix it :)
Also Zygarde gets the same error as Ash-Greninja when you try to enter different IVs: "Zygarde is only obtainable via event - it needs to match one of its events" but he isn't IV locked to my knowledge
As a side note, changing his ability to Power Construct from Aura break only yields the "Zygarde's ability Power Construct is banned by [Gen 7] OU"
BUT! having a shiny and Power Construct Zygarde leads to the following errors:
- Zygarde's ability Power Construct is banned by [Gen 7] OU.
- Zygarde is only obtainable via event - it needs to match one of its events.
So! It seems to me that Showdown thinks the only ability Zygarde can have is Power Construct, which isn't true, since it has Aura Break by default at both 10% and 50% and only gets Power Construct when you have all 100 cells, which is at no point mandatory to construct a Zygarde.