sudo: user is not in the sudoers file

What the message looks like

$ sudo apt update
[sudo] password for anna:
anna is not in the sudoers file.  This incident will be reported.

The user is not a member of the sudo group. On Debian the first user is often not added when a root password was set during the installation.
The user was added to the group, but the current session still has the old group list. A new login is needed.
The line %sudo ALL=(ALL:ALL) ALL was removed or changed in /etc/sudoers or in a file in /etc/sudoers.d/.
A user name rule uses a different spelling, for example another domain name or case.

Which groups does the user have in this session?
id
Who is in the sudo group?
getent group sudo
Which sudo rules exist? (needs root, use su)
su -c 'grep -rvE "^(#|$)" /etc/sudoers /etc/sudoers.d/'
Is the sudoers syntax valid?
su -c 'visudo -c'

Become root with su - (needs the root password) or use another admin account or the console.
Add the user: usermod -aG sudo anna. Keep the -a, otherwise the user loses all other groups.
Log out and in again, or start a new SSH session, then check with id.
To change rules, edit only with visudo or visudo -f /etc/sudoers.d/name. It checks the syntax before saving. A broken sudoers file can lock you out of sudo.
If sudo is not installed at all, install it as root: apt install sudo.

It is written to the system log, usually the authentication log or the journal. It is not sent to anyone automatically. An admin can find it there.

Group membership is read at login. An open terminal keeps the old list until you log in again.