Adressbook and Payment Methods Messed up

This is definitely not a "major" bug unless you're able to provide reproducible steps on a clean install.
Do you allow multiple "customer" profiles? Check the profile type settings.
The add payment method form probably shows you the billing profile form for the billing profile to attach to the payment method.

Thank you for quick response!

I am in the process of upgrading my D7 to D10. I used ubercart in D7, so I am new to commerce. I tried to find where I can set allowing multiple "customer" profiles, but I didn't find. Where is the setting for allowing multiple customers?

under People->profiles, I do see quite some customer records, more than one for adm. I believe they came from order testing. During check-out process, I can select an address or create a new one, so I guess multiple customers are allowed.

When saving the address record under adding payment method, there will be an error:

TypeError: Drupal\commerce_paypal\Plugin\Commerce\PaymentGateway\Checkout::createPaymentMethod(): Argument #2 ($payment_details) must be of type array, /chroot/home/aab0448d/86038db258.nxcli.io/html/modules/contrib/commerce/modules/payment/src/PluginForm/PaymentMethodAddForm.php on line 83 in Drupal\commerce_paypal\Plugin\Commerce\PaymentGateway\Checkout->createPaymentMethod() (line 842 of /chroot/home/aab0448d/86038db258.nxcli.io/html/modules/contrib/commerce_paypal/src/Plugin/Commerce/PaymentGateway/Checkout.php).

Go to "admin/config/people/profile-types".

TypeError: Drupal\commerce_paypal\Plugin\Commerce\PaymentGateway\Checkout::createPaymentMethod(): Argument #2 ($payment_details) must be of type array, /chroot/home/aab0448d/86038db258.nxcli.io/html/modules/contrib/commerce/modules/payment/src/PluginForm/PaymentMethodAddForm.php on line 83 in Drupal\commerce_paypal\Plugin\Commerce\PaymentGateway\Checkout->createPaymentMethod() (line 842 of /chroot/home/aab0448d/86038db258.nxcli.io/html/modules/contrib/commerce_paypal/src/Plugin/Commerce/PaymentGateway/Checkout.php).

PayPal checkout doesn't support saving/tokenizing payment methods (card on file), do you only use PayPal on your site?

Currently yes, I only have PayPal on site. What other methods should I use to avoid this problem?

Thank you!

Is this feature useful on your site? Do you want people to save their cards outside of checkout? The problem here is the way commerce_paypal was designed, there isn't much we can do in Commerce core to fix this.

I have disabled this feature for my users. Saving and editing credit card information is a popular feature for most sites. It would be nice if commerce_paypal provides it. But if it is out of your hands, that's fine. It would be nice if you could auto turn it off if PayPal is the only payment configured.