network forward create is meant for additional IP addresses that then point to instances, like floating IPs in the cloud.
In your case, as you seem to want to forward a port on the host to an instance, what you want instead is a proxy device.
incus config device add YOUR-INSTANCE port-8000 proxy listen=tcp:0.0.0.0:8000 connect=tcp:127.0.0.1:8000
This will make any connection to your host’s IP addresses on port tcp/8000 to be forwarded to the instance.