The "You must fill out the penname and email fields. Please try again" error typically gets triggered if you included a underscore or space in your username (as an example, if I were to sign up with 'Diamond April' instead of 'DiamondApril.')
You can actually change your username to include a space/underscore once the account is already registered, so if you're including a space/underscore at signup, I recommend signing up without it, then adding it later on by changing your username in the Account area. Using this method should work, at least until a more extensive site overhaul can be done.
Of course, this error might be more specific to you... but in order to idenity that, I would need further information regarding what exactly you're entering upon sign up. What email host (gmail, outlook, etc.) are you using? What username exactly are you attempting to register for? Does it include a space?
Regardless, if you're still encountering this issue, feel free to send me an email at diamondapril at lunaescence.com. You could send me the username or email you're attempting to use, and I can possibly set the account up for you.