Insufficient permissions/scopes on RESTful API

[jokoo353]

Hello to you. I'm new to eBay's RESTful API and would like your assistance to help me understand something.

I need to perform 2 actions via the RESTful API:

1. Configure Platform Notifications (push) on a web server
I tried to add a URL via developer.ebay.com, but when clicking on the "save" button of the "Platform Notifications (push)" form, I get this error: Something went wrong. Please try again later.

I then tried to use the "(Commerce Apis/Notification Apis) createSubscription" function with these parameters:

{
     "topicId": "ITEM_AVAILABILITY",
     "status": "ENABLED",
     "destinationId": "[my_already_created_destination_id]",
     "payload": {
         "format": "JSON",
         "schemaVersion": "1.0",
         "deliveryProtocol": "HTTPS"
     }
}

but then I get this error:

{
   "errors": [
     {
       "errorId": 195011,
       "domain": "API_NOTIFICATION",
       "category": "REQUEST",
       "message": "Not authorized for this topic."
     }
   ]
}

2. Obtain a product from the catalog

I tried to use the function "(Commerce Apis/Catalog APIs) getProduct/[my_epid]", but received this error:

{
   "errors": [
     {
       "errorId": 75015,
       "domain": "API_CATALOG",
       "category": "REQUEST",
       "message": "Insufficient permissions to fulfill the request."
     }
   ]
}

In both cases, it seems to be related to permissions/scopes. However, my access token was generated with all the available scopes. Is it possible to add others? If so, which ones should I add in order to succeed with my 2 API calls and how to add them? I looked in the documentation, but I can't find what I'm looking for.

Thanks!

* I can make calls to other endpoints without problem e.g. "(Sell APIs/Inventory API) getInventoryItems"
* I get the same error messages using the API Explorer tool or my own application.
* The URL of "my_already_created_destination_id" is publicly accessible and was created via the "Marketplace Account Deletion" form in the developer.ebay.com interface. Saving this URL was validated the first time with the challenge_code. However, if I click "Send Test Notification", I receive the following error:

Notification delivery failed with HTTP status code 404 from [my_publicly_accessible_url]. Please ensure that the marketplace account deletion notification endpoint is ready to receive notifications.

So I'm also not sure that this section is configured correctly either.