Hiraku can't use magic because of the farming tool
The omnipotent farming tool is a replica of Gryme, a weapon that would ordinarily drain away the user's life force, killing them instantly. Hiraku's body is unaffected because he was also granted a "healthy body", which can offset the drain to his life force.Gryme may be draining away his mana as well, leaving him unable to use magic, unlike everything else in the setting. If he didn't have the tool, he might be able to use magic just fine.
