Some customers found that Captcha is not showing up in CMS 12.
StepsThe Captcha element requires sites to use the session so you may need to add a session config into Startup.cs file.
Please add the code as below:
services.AddSession(); //add this to method ConfigureServices(IServiceCollection services)
app.UseSession(); //add this to method Configure(IApplicationBuilder app, IWebHostEnvironment env)