Print label issue while using the sandbox account using API.

Hello Guys,

I am using the sandbox account and then creating the shipment using API. Creating the shipment is working fine but finalizing it by creating Print label is not working and showing the Below error:

"undefined method 'map' for nil:NilClass"

Please take a look on the screenshot too https://prnt.sc/me3d2o

Is that mean that we have to use the real money for testing the label print functionality in testing account?

Please clear the doubt.

Thanks