By directly calling the ACS API - requires Basic authorization header
Through Adobe.IO - requires you to create an integration and generate JWT token
From the explanation you have shared here, the URL endpoint used suggests that you are going for method 1 i.e. calling API directly but then you are generating JWT token which is really not required here. It is required in method 2.
For method 1 you need to define a Basic Authorisation header which is missing in your case. It is very easy to generate it and fix the problem for you. However, it is a public forum and the steps beyond this point are risky to share on this forum with you.
Can you please log a support ticket and we will assist you over there with the missing point.
Please be aware that Engineering doesn't recommend using direct API calls to ACS endpoint. Recommended approach is to make use of Adobe.IO