Electrum on tails is outdated.

In order to send BTC from electrum on tails, you have to either

1) download and install electrum on the persistent storage. This will allow you to have this version of electrum between several boots, or
2) download electrum on another external storage (e.g. usb), mount it inside tails and use that version of electrum.

In both cases, make sure to verify the pgp signature of electrum before installing (installer) or running (appimage) it.