When you use filter in this case, it's checking for users that are the same as ("Joao Claudino", "1240", 2). There aren't any in the list, so filter returns []. You can't use head on [], because there's no first element.

Other notes about your code:

if x then True else False is the same as just x.
The otherwise case for userRegistration can never be reached.

I would write it more like this:

checkUser user userList = elem user userList

userRegistration user list
| checkUser user list = list
| otherwise = user:list