Click on All resources and find your Azure AD B2C tenant and click on it.
Click on Azure AD B2C Settings
Choose your account and click on Azure AD B2C Settings.
If you get this error you have to create new Azure AD B2C tenant because the b2c-extensions-app is created automatically as part of creating an Azure AD B2C tenant. It should always be created. If you create a new tenant and this app is not present, you should open a support case so that the Azure AD B2C team can look into this.
Now I have a working Azure AD B2C Tenant
Click on Applications
Click on Add
Provide Name, click on Yes for Include web app and Allow implicit flow. Then specify reply url and App ID URI. I am going to explain all this later.
Click on Create.
Remember this APPLICATION ID, this is called client id in the config files we are going to use later.