SCIM Create User "License limit has been reached" in Sandbox

I tried to create a user using the SCIM v2 API. While I have only one user in my sandbox account and the "create extension" API works fine, creating the user with SCIM causes the error message, that license limit has been reached.

Are different licenses needed for SCIM usage? How can I test user creation with SCIM in the sandbox environment?

curl --request POST 
  --url https://platform.devtest.ringcentral.com/scim/v2/Users 
  --header 'accept: application/json' 
  --header 'authorization: Bearer U0pDMTFQMDFQQVMwMHxBQURRU2hmajRvbG8yb3U2UzZ3SW1FVEhncFlieHJRRjg0TTNGdDdJTlhUZ3FhNUVPMmQzTkk1VEEzZ0VDenJyeHpfRUZZbF9DemlIdFF2SlZIWkJCU29oOTNRZDlVTmtDRmc4YVZ6NzlJOHR5Q1pWNlU1RUp3OXZ2ZU9oTFVHUWwtTWluY1kwLUlFLUN0WUNrYnlCSHR5bVpRcjE1eHo4a21VU3RNOUdQTHBHQTQwaHJuSE9xdnNqbDFtQnpISlR5UXw0ZVh3VWd8b1BHNjFwc29MWTctWm1MdEpUUGdZd3xBUQ' 
  --header 'content-type: application/json' 
  --data '{"active":false,"name":{"familyName":"Allen","givenName":"Barry"},"emails":[{"type":"work","value":"barry.allen@acme.com"}],"externalId":"A98765","schemas":["urn:ietf:params:scim:schemas:core:2.0:User"],"userName":"barry.allen@acme.com"}'

{
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:Error"
  ],
  "status": "403",
  "detail": "License limit has been reached"
}