Everything looks ok to me, if it's a public app you will need to go through the auth process (re-install the app) if you updated the scopes after the app was installed. If that isn't the issue the only other thing I can think of is that your attempting to make this call to a portal that does not have Marketing Hub Enterprise.
You'll notice from the documentation that it mentions "access to the data from this endpoint will require a marketing hub enterprise subscription".
Everything looks ok to me, if it's a public app you will need to go through the auth process (re-install the app) if you updated the scopes after the app was installed. If that isn't the issue the only other thing I can think of is that your attempting to make this call to a portal that does not have Marketing Hub Enterprise.
You'll notice from the documentation that it mentions "access to the data from this endpoint will require a marketing hub enterprise subscription".