HELP!!! I’m unable to create an API USER

I tried creating an api user in the postman by sending a post request on postman to this link https://sandbox.momodeveloper.mtn.com/v1_0/apiuser. The post request contained in my header X-Referene-Id(a random uuid generated by a uuid generator), Ocp—Apim-Subscription-Key(which is the primary key gotten after subsrcibing to a product, in my case I subsricbed to the collection widget producr) and Content-type(whose value was application/json). After doing all of this and sending the post request, I am getting a status code of 401 “saying Access Denied due to invalid subscription key. Make sure to provide a valid key for an active subscription”. I have confirmed multiple times that I use the primary key and i even tried out the secondary key but same thing, I have tried to open another account to subscribe to a new product, yet still the same thing. I will sincerely appreciate it if this can be attended to as soon as possible