Just to be clear I’m a fellow developer, not Shopify staff, so I can’t enable anything on your store. Only Shopify can do that.
Add your app’s client ID next to the domain, since the domain alone isn’t enough for them to act on. For precedent you can point them to: Physical inventory not accessible
And one thing you can check yourself: does a Purchase orders section actually appear in that dev store’s admin? If not, the “permission to view purchase orders” part of the error is failing too, which is worth telling them.