Non-root users cannot change their passwords on the web UI. This is because the "confirmation-password" parameter has not been set in password.php at this POST statement creation:
$post = array(
'userid' => "$username@pve",
'password' => $new_password
);
(line 56 in password.php"
Should be a simple fix, just have to capture the confirmation password in the web app in a new text box field. Will try to get to it at some point.
EDIT: To add more context, this seems to be from a Proxmox API update (based on the fact that the password change functionality worked in the video tutorial series)