I believe the problem is that your account is too new and bitcointa.lk hasn't sync the member list to that point yet.
I have checked with other new accounts on
https://bitcointalk.org/index.php?action=mlist;sort=registered;start=0;desc and the message "We don't know this username. Register it on Bitcointalk.org first." appears.
I'm not sure what you mean 100%, but I don't know if you can just sign up to that site and not this one. I'd just use the account you have now here on this site and not bother with the .lk for now until they sort the issue out.
You can't register an account on bitcointa.lk, but just link an existing bitcointalk account.