Microsoft Identity Client Error "An error occurred while sending the request"

Hello,
I want to make Azure AD B2C with theirs module Microsoft.Indetity.Client. But when I try to implement their sample. After I go through this line
" AuthenticationResult ar = await App.PCA.AcquireTokenAsync(App.Scopes, GetUserByPolicy(App.PCA.Users, App.PolicySignUpSignIn), App.UiParent);"
after a few seconds, I get the following error: "An error occurred while sending the request". What is wrong here? Does anyone have a clue?