By default sudo is not installed on Debian, but you can install it. First enable su-mode:
su -

Install sudo by running:
apt-get install sudo -y

After that you would need to play around with users and permissions. Give sudo right to your own user.

usermod -aG sudo yourusername

Make sure your sudoers file have sudo group added. Run:
visudo to modify sudoers file and add following line into it (if it is missing):

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL

You need to relogin or reboot device completely for changes to take effect.

Sudo is probably not installed or not in your path

1. check to see if you are root in this case sudo is not needed unless you are trying to impersonate another user. just run your command without sudo mv composer.phar /usr/local/bin/composer

2. See if sudo is your path by running which sudo or echo $PATH. If sudo is not in your path, your path variable might be broken. You can try testing this by executing a common location for sudo /usr/bin/sudo or running locate sudo | grep bin to attempt to find its location.

3. If you know that sudo was installed, or your path looks broken, try fixing your path. Check your distribution's env file (/etc/environment in ubuntu) to make sure that it is formatted correctly (script commands are illegal in this file)

4. If you are not root and you want to run a command with root prvileges then you must install sudo. But if you don't have sudo and you are not root then you can't install it. In this case I recommend switching to the root user with su

5. If you do not have the root password and you own the machine, you can reset the root password with a tutorial such as https://askubuntu.com/questions/24006/how-do-i-reset-a-lost-administrative-password

6. After you manage to login as root install sudo with apt-get update; apt-get install sudosince you are using Ubuntu.

7. Verify the the name of your sudoers group with visudo and modify your sudoers file if you need to. https://www.digitalocean.com/community/tutorials/how-to-edit-the-sudoers-file-on-ubuntu-and-centos

8. if you have an existing sudoers group or you create one you can add yourself to the group. For example if your sudoers group is called sudo run usermod -aG sudo myuser. The sudoers group by default in Ubuntu based Linux is sudo. A sudoers group entry looks like this: %sudo ALL=(ALL:ALL) ALL

If you are trying to impersonate another user and cannot install sudo, you can still use su if it is installed and you have permission / password for the other user.

e.g. su someuser

In a terminal type:

which sudo

That should give you an output of something like:

/usr/bin/sudo

If that doesn't, then it's not installed on the machine. The only way to use the superuser account in that case is:

su -

It will ask for a password, but if you don't know the password for the root account you're not getting into the machine at that point.

$PATH is evaluated by your shell, so your check doesn't work as you expect it to.

/etc/sudoers is configured to replace your PATH with a default one.

sudo does not load a login shell environment before executing the command, so the default PATH from /etc/sudoers is used. su - does open a login shell, which involves loading /etc/profile. See man bash, section INVOCATION.

Just remove the PATH reset in /etc/sudoers. It's likely a rule called secure_path.

CentOS

In CentOS you can add PATH to the Defaults env_keep section:

Defaults    env_keep = "COLORS DISPLAY HOSTNAME HISTSIZE INPUTRC KDEDIR \
                        LS_COLORS MAIL PS1 PS2 QTDIR USERNAME \
                        LANG LC_ADDRESS LC_CTYPE LC_COLLATE LC_IDENTIFICATION \
                        LC_MEASUREMENT LC_MESSAGES LC_MONETARY LC_NAME LC_NUMERIC \
                        LC_PAPER LC_TELEPHONE LC_TIME LC_ALL LANGUAGE LINGUAS \
                        _XKB_CHARSET XAUTHORITY PATH"

If sudo is not installed and you don't know the root password, the only way would be to attach your USB stick to another PC and clean it up.

If sudo is installed, then it may be a path setting problem. The "sudo" binary should normally be in "/usr/bin". You can search for it:

find / -name sudo 2>/dev/null

(the "2>/dev/null" part is to get rid of all "Permission denied" warnings you get in the result)

And then use the path:

/usr/bin/sudo rm -f ./nextcloud-bkp_2017*

If it works, you need to see what is wrong in "/etc/profile" file (where the PATH is originally set) and fix it.

It would help if you mention you RasPi and OS versions. And also what user you are logged on with.