Hello! Welcome to our world wide family WikiTree! Thank you for joining us.
You can not have a blank Last Name At Birth because the WikiTree ID is created using the LNAB. If http://www.wikitree.com/wiki/Velani-24
is the ancestor with no last name at birth, I would leave it Velani-24 and put a statement in his biography section that he had no last name at birth, but later generations usd Velani.
The other option is to change the LNAB to Unknown, but I think the first choice is better.
Are you indicating that there is a duplicate profile on the WikiTree with the wrong last name? If so, you should post a message on the bulletin board on that profile asking about sources for the incorrect surname and including you information and sources For the correct surname. After the surname is decided, if they are really two profiles for the same man, they must be merged. Remember, only one leaf on the WikiTree per person. Duplicates must be merged.