My USB stick is now full of backups, and I need to delete at least one to upgrade Nextcloud.
I cant.

I try `rm -f ./nextcloud-bkp_2017*`
and I get:

    rm: cannot remove './nextcloud-bkp_20171211.tar': Permission denied

So I try `sudo rm -f ./nextcloud-bkp_2017*`
and get:

    -bash: sudo: command not found
I then try `su`
And, it asks for a password.

I have tried every password I know, and some I didn't.

It always comes up:

    su: Authentication failure
I even try (without knowing if it's the correct app):

    apt install sudoer
and, of course, get:

    E: Could not open lock file /var/lib/dpkg/lock - open (13: Permission denied)
    E: Unable to lock the administration directory (/var/lib/dpkg/), are you root?

Is there anything I can do?

Where did sudo go?