Tell PMPro allow weak passwords on the change password and reset password forms.

pmpro_allow_weak_passwords.php

/**
 * Tell PMPro allow weak passwords on the change password and reset password forms.
 * Requires PMPro v2.3.3+
 */
add_filter( 'pmpro_allow_weak_passwords', '__return_true' );

This recipe is included in the blog post on "Allow Members to Create Weak Passwords During Reset or Profile Update" at Paid Memberships Pro here: https://www.paidmembershipspro.com/allow-members-use-weak-passwords/