Thank you for additional info.
However, I’m unable to reproduce the issue on my test server. I’ve installed clean WordPress 5.4 on a server with PHP 7. Switched the language to Japanese, installed Stripe Payments, Subscriptions and ACF addon. Then enabled coupons and created a coupon without any issues. I have even created a coupon with discount code on Japanese (25オフ) and it also worked just fine.
Can you try to delete Stripe Payments plugin and install it again? Don’t worry, your settings will not be deleted.
Also, if this won’t help, can you try to disable ALL plugins BUT Stripe Payments, Subscriptions and Additional Custom Fields and see if this helps?