AADSTS50126: Error validating credentials due to invalid username or password

Question

I am quite new in Azure and B2C.

Up today, I am familiar with Tenants, Directories, Subscriptions, UserFlows AppRegistrations and have been able to run this sample.
I am nor really familiar with scopes, API permissions and ROPC.
What I am trying to do is to insert B2C into my developed webApps. The idea was to include it as this sample. My appsettings:

{  
  "Authentication": {  
    "AzureCloudInstance": "AzurePublic",  
    "AadAuthorityAudience": "AzureAdMyOrg",  
    "TenantId": "3d4b9781-2052-466a-b45c-xxxxxxxxxxxxxx",  
    "Domain": "xxxxxxxxxxxxx.onmicrosoft.com",  
    "ClientId": "9e99f9ee-1f6d-4668-8f4d-xxxxxxxxxxxxxx"  
  },  
  "WebAPI": {  
    "MicrosoftGraphBaseEndpoint": "https://graph.microsoft.com"  
  }  
}  

The result:

Error: AADSTS50126: Error validating credentials due to invalid username or password.  
Trace ID: 9f636bfe-44ab-4bad-92ef-b5b8f6dd3600  
Correlation ID: 6104315c-c8b3-4f5b-92e4-510f781cc8cc  
Timestamp: 2021-03-19 14:51:03Z  

I tried also this:

var url = $"https://login.microsoftonline.com/{Domain}/oauth2/token";  
var values = new Dictionary<string, string>  
{  
 { "resource", "https://graph.microsoft.com" },  
 { "grant_type", "password" },  
 { "client_id", CleintId },  
 { "client_secret", Secret },  
 { "username", Username },  
 { "password", Password },  
};  
  
var request = new HttpRequestMessage(HttpMethod.Post, url)  
{  
 Content = new FormUrlEncodedContent(values)  
};  
//request.Headers.Add("Content-Type", "application/x-www-form-urlencoded");  
var response = await client.SendAsync(request);  
  
var responseString = await response.Content.ReadAsStringAsync();  

and this:

var url = $"https://login.microsoftonline.com/{TenantId}/oauth2/v2.0/token";  
var values = new Dictionary<string, string>  
{  
 { "client_id", CleintId },  
 { "scope", "https://graph.microsoft.com/user.read" },  
 { "username", Username },  
 { "password", Password },  
 { "grant_type", "password" },  
 { "client_secret", Secret },  
};  
  
var content = new FormUrlEncodedContent(values);  
  
var response = await client.PostAsync(url, content);  
  
var responseString = await response.Content.ReadAsStringAsync();  

I always get AADSTS50126: Error validating credentials due to invalid username or password.

In any case, the password is correct, I am able to log in using 1-WebApp-OIDC/1-5-B2C example.
I have registered two different application, one for 1-WebApp-OIDC/1-5-B2C and one for console-up-v2

What do I miss?

I took also a look to this guide but no idea about TrustFrameworkExtensions.xml and where to find it.

EDIT:

I tried followings:

var url = $"https://{TenantName}.b2clogin.com/{TenantName}.onmicrosoft.com/B2C_1_{ROPC_UserFlow}/oauth2/v2.0/authorize";  
            var values = new Dictionary<string, string>  
            {  
                { "username", Username },  
                { "password", Password },  
                { "grant_type", "password" },  
                { "scope", $"openid {ClientId} offline_access" },  
                { "client_id", ClientId },  
                { "response_type", "token id_token" },  
            };  
  
            var request = new HttpRequestMessage(HttpMethod.Post, url)  
            {  
                Content = new FormUrlEncodedContent(values)  
            };  
            var response = await client.SendAsync(request);  
  
            var responseString = await response.Content.ReadAsStringAsync();  
            Console.WriteLine(responseString);  

Get:

var CONTENT = {"contact-none":"Your administrator hasn't provided any contact details.","contact-number-label":"Telephone","contact-email-label":"Email","error-title":"Sorry, but we're having trouble signing you in.","error-help":"We track these errors automatically, but if the problem persists feel free to contact us. In the meantime, please try again."};