However, this does not appear to be working at all and the following error is returned when attempting to create a storefront access token from a custom app:
"errors": "App must have a channel record to create a storefront access token."
The error makes some sense as the custom app is not a sales channel (because custom apps are not able to be a sales channel) and personally I was curious how this was going to even work... but regardless it is supposed to work.
Shopify partner support was not able to provide any insight other than agreeing that it did seem like an issue, I'm expecting a follow-up email from them once they find out more.
Any experience or information on how this is supposed to work would be appreciated.
I also hit this wall. I did end up getting around it, finally, at least for a custom app I created via the partner dashboard that I am, for now, using only for exploring shopify APIs. Here is what I did:
Uninstall your app from your store (may not be necessary but it is the step I took)
Go to your partner dashboard. Find your app. Go to the app settings page for your app. Click the "Enable Storefront API" button.
Ensure you request the necessary "unauthenticated_*" scopes during your installation handshake.