The losing carrier has rejected the port request for the number stating that the phone number cannot be ported.
The most likely case here is that the phone number uses services on the losing carrier that prevent it from being ported. For example, in Japan both rental phone numbers & public phone numbers cannot be ported.
In rare cases, the number may have been incorrectly typed by Twilio's Portability API and the requested number is not actually portable.
Check with your current carrier to determine if you are using any services that would prevent the phone number from being ported to Twilio. If you can remove those services you may still be able to attempt a port.
Otherwise, please reach out to Support at porting@twilio.com and they can help to troubleshoot the rejection further and find alternatives for you.