You are doing everything right just one thing you need to request order permission from your partner account first before using this read_all_orders scope in your app .toml /.env file. Check this link below for reference.
I’m having the same problem. My custom admin app was designed on the shopify store, so there is nothing in my partners account to request. It’s not going to be used anywhere but the one store.
I’ve tried following numerous pages of docs, but no luck…the site owner tried using this section to give me access to read_all_orders and couldn’t find it.
Failing that, is there a way to get a csv dump of all old orders? This is only a one-time need, so I could try to do it that way.