Hi there,
I'm sorry to hear you are having trouble shipping!
This error is not generated by ShippingEasy, but by UPS directly. Since it is UPS' API we unfortunately do not have the option to override this.
I suggest you contact UPS to verify that your account is active and you are authorized to ship using the selections you made in Ready to Ship. It is possible your account is not eligible for your chosen shipping method.
When you contact them, let them know you are using their API to create shipments, and also notify them of the shipping selections you made so they can compare.
All of their contact information can be found here.
Warm regards,
Taylor Painter