hostnamectl; # tested on
  Operating System: Debian GNU/Linux 10 (buster)
            Kernel: Linux 4.19.0-8-amd64
      Architecture: x86-64

# find the umount binary
/usr/bin/umount

# this spelling problem
# can drive one mad
# if in a hurry
ln -sv /usr/bin/umount /usr/bin/unmount

# also:
sync; # before umount?

imho this should be default on all (L)unixes.

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