I follow all instructions given in below two links.

1. Enable external features  

2. Nintex External Forms Online Service for 2016 

Still, I can not able to launch my form as public, I set my site and list already available for anonymous access. 

After all configuration options form "External form setting" is not enabled (Please go through Attached image),

Can anyone tell me the problems or steps how can I achieve my requirements?

The environment I use is Sharepoint 2019.

Thanks in Advance.