I'm trying to recover the access token via the Instagram Basic Display API but when trying to authenticate the test user I get this error:
{
"error_type": "OAuthException",
"code": 400,
"error_message": "Invalid platform app"
}
I expect to see the app authorization screen

Felice!
When setting up an Instagram app, you should use the platform-specific App ID and not the generic one set up on Facebook.
In your Facebook app Dashboard go to
Products > Instagram > Basic Displayand should see the Instagram App ID.Use that in your authorization URL and it should work.