IDNs only available in the following character sets:
A General-use JP domain name containing Kanji, Hiragana, and/or Katakana must consist of one to fifteen (1~15) characters, excluding ".JP" suffix. The number of characters in an ASCII JP domain name must be from three to sixty-three (3~63).
Please note that if ASCII, Kanji, Hiragana and/or Katakana characters coexist in a same label of a General-use JP domain name, each character will be counted as one, whether or not it is of double byte code or single byte code.
A full list of supported Japanese characters can be found on following website: https://jprs.jp/doc/rule/saisoku-1-wideusejp-furoku-4.html
Supported IDN Scripts
The following IDN scripts are supported by .jp.
Provide the LANGUAGE TAG corresponding to your desired IDN within the AddDomain command as X-IDN-LANGUAGE extension.
Get more information of "IDN tables", which represent permitted code points (letters) allowed for Internationalised Domain Name registrations in particular registries on IANA Repository of IDN Practices.