The gift card may have expired, check the validity of the gift card. Or, you could just wait for Bank to be free for a month.
Region changing for eShop use may be risky business. Even tho for region changing, most of the internal memory would be swapped out anyway, and duplicated apps has to be deleted, you may screw up someone and the server recognises your device as “modified” or “originally being Japanese”.
I say just wait for it to be free. Bank is gonna be gone soon anyway, no point having a subscription..