Ubuntu Install Error: "main: line 1758: /opt/pihole/gravity.sh: No such file or directory"

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To add to confusion, for me, adduser depends on the passwd package that contains useradd already:

pi@ph5:~ $ hostnamectl | tail -3
  Operating System: Raspbian GNU/Linux 10 (buster)
            Kernel: Linux 4.19.97-v7+
      Architecture: arm

pi@ph5:~ $ apt depends adduser
adduser
  Depends: passwd
[..]

pi@ph5:~ $ which useradd
/usr/sbin/useradd

pi@ph5:~ $ dpkg -S /usr/sbin/useradd
passwd: /usr/sbin/useradd