In order to add money to an account you need to follow the security restrictions in place. This is covered in the Battle.net Balance FAQ https://us.battle.net/support/en/article/battle-net-balance-faq#q1
How do I add to my Battle.net Balance?
When you log in to Battle.net Account Management, you are able to purchase Battle.net Balance funds through a variety of different payment methods, such as debit or credit cards or a variety of region-specific payment methods. Any amount you add to your Battle.net Balance will not be refundable (except as otherwise provided by law). Please note that depending on the payment method, it may take some time for the Battle.net Balance you purchase to become available to use. You will see which options are available when you log in to Battle.net to add balance.
Note: In order to add Battle.net Balance funds to your account, you must have a Battle.net Authenticator attached
The RMAH has some additional restrictions, so if you want to use PayPal in any way then you also need SMS Protect.
You can read about those restrictions here: https://us.battle.net/support/en/article/diablo-iii-auction-house-functionality#q16
Are there any account-security measures tied to the real-money auction house?
Players will be required to attach a Battle.net Authenticator or a free Battle.net Mobile Authenticator in order to charge up their Battle.net Balance via Battle.net Account Management or to receive the proceeds of a successful auction house sale in their Battle.net Balance. For more information about how to obtain an Authenticator, please visit this page.
Players who wish to use PayPal to charge up their Battle.net Balance or to “cash out” funds earned in the real-money auction house will be required to sign up for our SMS Protect authentication service in Battle.net. When you sign up for the Battle.net SMS Protect service, you will receive a text message on your mobile phone containing a code that you must enter on to verify your identity. Once verification is complete, you can then move forward with your transaction. After this initial verification, you will be prompted to enter a new SMS code from time to time when accessing a feature that uses PayPal. Note that standard SMS messaging rates apply -- check with your cell phone carrier for details.