Debian 9: can't use sudo

It seems you have been bitten by a bug in the Debian 9 installer, as described in this forum topic: http://forums.debian.net/viewtopic.php?f=17&t=133604

There's a workaround given in the (currently) last post in that thread.

As I recall, the sudo command is not configured in Debian unless there is no root password given on install. Formerly, the sudo command was not even installed by default in Debian.