When processing UPS orders, you might come across the "Hard 120603 - Invalid Package Reference Value" error.
This error happens when the Reference Numbers exceed the length of 35 characters when using UPS Mail Innovations.
It could also be a false flag referring to references, but actually related to any other field exceeding 35 characters.
To solve this, you can either:
a) Use shorter reference numbers
b) If that's not possible, remove or replace the reference as this article indicates: Using order References in your labels [Step-by-Step guide]
c) Check other fields in the order (Address, SKU, etc.) that can be exceeding 35 characters.