Unable to Generate API User – Invalid Subscription Key Error

I hope you are doing well. I need assistance with an issue I am encountering while trying to generate an API user in sandbox environment.

I have followed all the steps outlined in the documentation:

1. Signup For An Account – Completed

2. Subscribe To Our Products – Completed (subscribed to Collection Widget)

3. Manage Your Subscriptions – Subscription is active, and I have received both the primary and secondary keys

4. Generate API User and API Key – This step is failing
    

To generate the API user, I am using the following endpoint:

• POST https://sandbox.momodeveloper.mtn.com/v1_0/apiuser

Headers sent:
{
  "X-Reference-Id": "<uuid-v4>",
  "Ocp-Apim-Subscription-Key": "<Subscription-Primary-Key>",
  "Content-Type": "application/json"
}

Body :
{
  "providerCallbackHost": "webhook.site"
}
 

However, the API returns the following error:

Response:{  "statusCode": 401,  "message": "Access denied due to invalid subscription key. Make sure to provide a valid key for an active subscription."} I have verified that the subscription is active and have also regenerated the subscription keys, but the issue persists.  Could anyone advise on the correct steps to resolve it?