mona is not in the sudoers file. This incident will be reported

The usual way to give sudo rights to an account in Ubuntu is to add it to the admin and/or sudo groups, which are given sudo rights by default.

If you really want to modify your sudoers configuration, you should not edit /etc/sudoers directly, but add your local configuration in a separate file in the directory /etc/sudoers.d as described in man sudoers. This is to avoid the problem you just experienced: since your changes will not be in /etc/sudoers, you can safely upgrade it to newer versions without losing your configuration.

In order to modify system files if sudo won't work, you can go through recovery mode. You can then, for example, add mona to the sudo group with

usermod -aG sudo mona

Whoops, your "Y" accepted a new version of the sudoers file that obviously doesn't have you in it! In the future, a "D" to compare them would have been a better option, though it's also not clear that this should have happened in the first place: having your sudoers file replaced while installing software is not very professional. At this point, you'll need to log in as root (not yourself with sudo) and re-add yourself ("mona") to the Users section of the file.