Thanks for posting in the Shopify Community. This error can occur within our system for a few different reasons, one of which being that the address is invalid. I first recommend inputting both the sender address and the customer address into the UPS Address Validator tool. If both addresses are valid, you can get in touch with our live support team so we can investigate further. I'm unable to provide account specific support in the Shopify Community, but if you contact us through the Help Center, we'll be able to work on getting this resolved for you.
Please let me know if you have any other questions!
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