Install libpam-pwdfile by entering the following commands in the terminal:
sudo apt update sudo apt install libpam-pwdfile
PAM module allowing authentication via an /etc/passwd-like file
This PAM module lets you use an arbitrarily-named text file similar in structure to /etc/passwd to authenticate users. It supports libc's crypt plus md5crypt with a broken md5 and Digital UNIX's bigcrypt.