I've found a workaround.
I'll share it here.

Issue:
You can't add / save cards in your shopee account, both shopee app (Mobile) and website (PC).
1. Settings --> My account --> Banks & Cards --> Add New Credit Card
2. ShopeePay --> Top Up --> Payment Method --> Add New Credit Card
3. Card verification page shows up, instead of normal refundable RM1.00, it charges RM0.00 here.
4. You will received OTP from your card issuer. But after this step, it'll bring up error code 5.

Workaround:
1. PC: clear browser offline data and cached. Re-login.
2. Mobile: clear shopee app all data and cached. Re-login.
3. Shopping Cart --> Checkout --> Payment Option --> Add New Credit Card

It'll temporary save your card without asking for OTP before payment made.
Keep proceed. Then it will come to the OTP page, now it will charges amount RMX.XX instead of RM0.00.
Continue the payment and your card will be saved this time.

Please complete step 3. You must paid directly using the card, in that way only it'll save your card permanently.

Once you follow my workaround paid directly, it'll save your card permanently. That means also can topup shopeepay like normal again.

Check out Daily RM3 Cashback with ShopeePay on Shopee! https://shopee.com.my/m/shopeepay-daily-rm-...back?smtt=0.0.3

Just for fun, I still have a lot of credit at Lazada Wallet, top up Lazada Wallet got/won special rebate (rebate from Lazada)!