Error: #120412 UPS account number that was provided as the payment method is missing or invalid

The issue is that the UPS account # you are trying to use for the 3rd Party or Recipient Billing does not match that address information for that account in the UPS servers, or the customer has disabled their account for third-party/recipient billing. 

UPS Ready software like NRG's UPS Shipping Software is transactional and all ship information is verified by UPS before a label can be generated. If the UPS servers receive a mismatch of information, the transaction will be rejected.

The postal code required is usually the "billing" postal code that the customer has on file at UPS.

Double check the account number and associated postal code.  If the problem persists, then contact the owner of the UPS Account and find out the address they have on file with UPS. If the error still persists, then their UPS account rep should be contacted to ensure that the account is enabled to allow for 3rd party and recipient billing.