I can reload my Japanese Suica transport card on the wallet app. I don’t understand why it won’t work.

Hello,

These two articles mention using a Suica card and some requirements you may encounter:

Use Suica or PASMO cards on iPhone or Apple Watch in Japan - Apple Support

Add a Suica or PASMO card to Apple Wallet - Apple Support

Contact Apple; click here --> Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support

Foreign issue VISA cards are not currently working for Apple Pay In-App recharge with Suica and PASMO Wallet cards. Use Mastercard or AMEX.