Hi yamcake3821, Thanks for posting here in the Microsoft Community, it sounds like the Japanese input method didn’t install properly.
Try this, simply go to Settings then Time & Language then Language & Region then Remove Japanese completely after that Restart your PC then Re-add Japanese and make sure Microsoft IME is selected under Keyboards.
If it still says “unavailable,” check for Windows updates and install any pending ones.
Let me know, did you install the Japanese language pack with admin rights, or on a standard user account? That might help isolate the issue.
I hope this helps you out, let me know how it goes.
Chard