I prefer a four digit PIN so, ran into this issue, myself.
The PAM authorization checks for passwords is located in /etc/pam.d/common-password
You can open the file using File Manager elevated to root or by using sudo gedit...
But open the file, then move to this line:
password [success=2 default=ignore] pam_unix.so obscure use_authtok try_first_pass yescrypt
Change the word obscure to minlen=1
Save the file.
Then try changing your password as previously attempted.