I tried to reproduce the same in my environment and got the below results:
I created an Azure AD B2C application and granted API permissions like below:
Make sure to grant IdentityUserFlow.ReadWrite.All permission:

I generated an access token via Postman by using client_credentials flow by using below Parameters:
https://login.microsoftonline.com/TenantID/oauth2/v2.0/token
client_id=Your_client_ID
client_secret=Your_client_Secret
grant_type=client_credentials
scope=https://graph.microsoft.com/.default
Response:

By using the above generated access token, open a new tab in Postman and include the parameters like below:
In Authorization tab, select Type as Bearer Token and paste the access token like below:

In the Header tab, add Content-type: application/json and In the Body tab add like below:
{
"id": "testuserflow",
"userFlowType": "passwordReset",
"userFlowTypeVersion": 3
}

By using the below Endpoint run the query and user flow will be created successfully like below:
POST https://graph.microsoft.com/beta/identity/b2cUserFlows

In the Azure Portal, the testuserflow is successfully created like below:
