Sometimes, not enough chain data has been collected in order to estimate a suitable transaction fee.
Usually just waiting a few blocks will allow the fee estimate to complete, but you can try setting the txfee to 0.0001 or the fallbackfee to 0.0002. As fallbackfee is only a startup option requiring either
in the startup command, or
in the veil.conf, it's not likely to be convenient compared to waiting a few minutes or simply overriding the fee with the debug console and this command: