The trial extension is only available after the purchase of Audirvana (note that this extension is for a new account and it’s not avaiable in UK for Qobuz) but you can try either Qobuz or Tidal for a month with your trial. Have you connected your Qobuz and Tidal account in Audirvana?
After using your license key in Audirvana, you will need to disconnect the Tidal trial account you created and you will see the voucher button in Audirvana.