ok, the first thing to check is.... the person you are signed in as, do you have the correct permissions to see and manage developer portals?
There is a builtin role, roles/apigee.portalAdmin which gets apigee.portals.* permissions.
If you have portalAdmin role, or some other role like Owner or Editor, which includes those permissions, then you should be able to do the thing you are trying. In which case, your question looks like a question for the Apigee support team.
if you are signed-in to an account that does not have the portalAdmin role, and does not have the required portal permissions, then that would explain the 403 Forbidden message.
You can read more about the portal roles and permissions in the documentation page.
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