There are no sites in the config file on nitro add
It probably because nitro doesn't overwrite your host file. You may need to edit it yourself using nano (assuming you're on linux).
sudo nano /etc/hosts
you will need to provide

the IP address- that you can find using nitro info in the terminal or by opening the nitro-dev shell in multipass
the hostname- example.test or whatever else that may be

There is a github issue raised for it, see https://github.com/craftcms/nitro/issues/213