Is there a VPN active? Either on the phone or the network the Deck is connected to?

Make sure you’re using the correct Steam Guard code, check that the date and time on your Steam Deck are correct, and try switching networks (Wi-Fi or mobile hotspot) and restarting. If that doesn’t work, go into desktop mode and delete old tokens with rm -rf ~/.local/share/Steam/config/ssfn*, then restart and try logging in again. Sometimes you just need to wait a few minutes after multiple failed attempts or contact Steam Support to approve the device.