1) There should be at least one user in the Domain -
that is me site.webmaster@fll-innofest.com

2) Open sites.google.com  (original site https://sites.google.com/trumptech.com/gstest-customdomain/home)

3) Share the Sites with the user in that domain with the editor access

4) Ask the user to make a copy of the file once the copy is made, publish the site to public -
this site ( https://sites.google.com/fll-innofest.com/copy-of-gspeak-test/)

5) Once it is published, map a custom URL to the site. (at admin.google.com - Apps - Sites - Custom URL +)

Please find the article: https://support.google.com/a/answer/7392423