What does this mean?

[gregarreaper]

※半角英数小文字4～16文字以内で入力してください
※登録後の変更はできません

I tried to make a user name called DeathByDonut301 but it wont accept it can anyone tell me what I'm doing wrong or give me a user name that would work.

[kurisuto]

"Use between 4 and 16 lower case half-width ASCII characters". (well, maybe there's another way to say "half-width ASCII characters", but to tell you the truth, most computer talk is gibberish to me, whatever the language)

"You may not modify (your user name) after your registration."

[gregarreaper]

problem is when ever I try to use a username it wont allow me to use it. The games in japanese so i tried using IchiShinobiJu in katakana then another in Romanji DeathByDonut301 but both dont work might you know the problem. Latale.jp

[kurisuto]

Well, I said I computer talk was gibberish to me, but at least I try to look up things. Try "lower case" and "ASCII characters"...