[Payment App] Get permission error in merchant on boarding flow

[EddieLuong]

Hi all,

I want to build a payment app to add my payment method to merchant's site checkout as a new payment option. I'm working on "merchant on boarding" flow and follow this document 

I finished the following steps:

1. Created partner account and has been approved to be Shopify's payment partner.

2. Created a development store for testing.

3. Created a publish app and got the API key and API secret key.

4. Created an extension, it's already been approved by Shopify and published.

When I redirect user to the URL https://{my_development_store}.myshopify.com/admin/oauth/authorize?client_id={my_api_key}&scope=write_payment_gateways%2Cwrite_payment_sessions&redirect_uri={my_redirect_uri}&state=b7f146e60403764ad097e29319ced0dd to ask the merchant for permission, I got the error below (i'm testing on development store)

Please let me know what I missing?

Appreciate your advise on this. Thanks