Maybe because of the following:

• It is not a cname record

• Make it www.yourwebsite.com

• Did you add WWW to the name section?

If none of these work, I usually just refresh the page and enter the domain again until it works lol

make sure the domain is like www.example.com (for both repl.it and your DNS)

Well, that feature is removed, but i think it still exists in a few repls. IF you put it properly and all. Then refresh, or make a new repl. if all fails, report to bugs. click here