The IAP server API that you used is for verifying the purchase token for the non-subscription order services. That is why you receive that message.
You should use subscription service API instead - {rootUrl}/sub/applications/v2/purchases/get
You can read more about the api documents here.
Here is an example:
POST /sub/applications/v2/purchases/get
Content-Type: application/json; charset=UTF-8
Authorization: Basic QVQ6Q1YzQ1NPbVlBaGwrZUtRWExtMTBVV2pyWXZHTVF4MmYvcVMya1B0ZElLY3UwaFJrdFNTMmxwdk1FQkIyYldXWGt0REVaR3I4UjFUTTRLMVlmNXdwWU80RG04THdXQWxjaFhEYjBMUjBNTUJtWnFYcGFtazc3THN3UnFJbkhHK28xekdqRzNSMg==
Accept: application/json
Content-Length: 171
{
"purchaseToken": "00000173741056a37eef310dff9c6a86fec57efafe318ae478e52d9c4261994d64c8f6fc8ea1abbdx5347.5.3089",
"subscriptionId": "1581789719266.D40972AC.3089"
}
Please also refer to this document - Verifying the Purchase Token for the Subscription Service