Access /portals/ API via OAuth2 authentication

Access /portals/ API via OAuth2 authentication

I'm trying to use the /portals/ API to fetch the list of portals that I have access to.
I'm using the following API call:-

curl -X GET \
https://projectsapi.zoho.com/restapi/portals/ \
-H 'authorization: Bearer <my-access-token>' \
-H 'cache-control: no-cache' \
-H 'content-type: application/json' \

However, I'm getting :-
{
"error": {
"code": 6500,
"message": "General Error"
}
}

Any pointers on what I need to correct?