Can't access or enumerate sub-domains

Hello everyone hope y’all having great day
I’m completely newbie to CTFs and I’ve done some easy ones before today I tried to do Devvortex box and firstly I did nmap scan which went normal then I did directory scan and also went normal but when I came to the sub-domain scanning there were no results(I used gobuster) I searched for writeups to see if this is normal but I found there was a sub-domain called (dev) I tried to access it but I couldn’t unless I added it to my /etc/hosts is this ok?and why I didn’t get it during the scanning process?
(I mentioned earlier I’m a complete newbie)