Thank apple for complicating app signing beyond reasonable...

1. make sure you have a valid Apple developer license
2. make sure you have valid certificates/provisioning profiles for your app (and you have registered the bundle id on their developer portal)

Try ns prepare ios --release then open platforms/ios/<appname>.xcworkspace in XCode, and check the Signing details, if anything is missing - this will allow you to generate a new cert/provisioning profile right in XCode. If that checks out, try Product -> Archive within XCode and see if that succeeds. You can upload this directly to TestFlight for release after archiving from the window that pops up (Organizer).

If all went well through XCode, then the CLI command should work as well

ns build ios --release --for-device