I'm under the impression it's because it's flagged as Japanese, yet it's name is in English.
Both are the exact same Mewtwo, except one has it's name in English, and other in Japanese.
(Obviously, the Legal one is the Japanese one; Both were flagged received as JPN btw)
Additionally, I just created another Mewtwo, with it's Nickname in English, and also flagged as received in English.
It appears legal.
TLDR; the error you're entering is because you mismatched the language byte and nickname. Working as intended.