Hi @Luke, did you manage to resolve this ? @Paige-Shopify I have a very similar issue where I have created an app with two extensions(1.an admin UI extension that surfaces a UI for merchants to create discounts, 2.a Function that validates the discount and either applies or rejects it based on certain properties).
Like Luke, I am trying to send custom rejection messages, but the rejectable property is always false. Any insight would be greatly appreciated! I have posted my own thread about this here.