Hi @Paulina Bravo,
Sorry for the late reply, it took me a while to discuss and look up the information with my MS Graph colleagues. Your problem is probably caused by insufficient permissions.
First let's go back to the API Permissions page of the app you registered in AAD.
First, if you choose MS Graph:
You need to give the appropriate access to site for the apps you have registered. Here lists all Microsoft Graph permissions, you need to add site-related permissions.
Second, you are using the SharePoint REST API for the request. You should use a request with a graph. The request URL should look like this:
https://graph.microsoft.com/v1.0/sites/{site-id}/drive/items/{item-id}/children
I'm attaching the API documentation for graph here: Working with SharePoint sites in Microsoft Graph
Reminder: Document operations on SharePoint document libraries are categorized under Drive
Second, if you want to keep your request URL, then you should select SharePoint and give the relevant SharePoint permissions.
I'm attaching the API documentation for SharePoint Rest API here: Microsoft Graph REST API v1.0 endpoint reference.
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