Command 'yum' not found, but can be installed with: apt install yum code example

Example: sudo: yum: command not found

After simple checks I've asked you in answer comment:

[root@Backup yum.repos.d]# yum
-bash: yum: command not found
[root@Backup yum.repos.d]# which yum
/usr/bin/which: no yum in (/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/root/bin)
[root@Backup yum.repos.d]# echo $PATH
/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/root/bin
[root@Backup yum.repos.d]# rpm -q yum
package yum is not installed
we can verify that you don't have yum installed in your system.

Please install it via rpm and you'll be able to launch it.