Here is the Udemy text
What name are they trying to use? I have searched under User from the Admin tab and no results come up for “Chase” or “Lang”. Searching for @chase brings up a user but the profile picture is vastly different to the one in your post.
Probably going to need at least the username they are trying to use.
The only other thought is whether Discourse holds partial registrations, e.g you start doing it but never get around to completing it but effectively take the username (or reserve it if you like), if so, its possible that they may have taken the username themselves?
You will have better visibility on the options than me as an admin 
That said…
Just been in under Admin and searched for the users there… there are two accounts for that person by the looks of it, two different email addresses - one of them was emailed 6 days ago… might be worth checking the spam folder…
Just pm’d you with a screenshot of the email addresses for this user…
Update
Is this all ok now @Michael_Bridges, I can try contacting the individual via the one email address available (screenshot in your PM) - but it will be coming from my somewhat more random looking GMail account rather than a nice shiny GameDev.tv one
I have re chased Chase today to see if he is sorted 
/insert Benny Hill them tune here
@Michael_Bridges d o we know if it’s this guy? https://community.gamedev.tv/admin/users/796/creativesnowman.
If so please CLOSE this topic (spanner at bottom left), if not can we still find them?