custom-registration-error-message.php

<?php
/**
 * Customize the registration error message.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function custom_registration_error_message( $errors, $sanitized_user_login, $user_email ) {
	if ( isset( $errors->errors['username_exists'] ) ) {
		$errors->errors['username_exists'][0] = 'This username is already in use. If you have an account, please log in to renew your membership.';
	}

	if ( isset( $errors->errors['email_exists'] ) ) {
		$errors->errors['email_exists'][0] = 'This email is already in use. If you have an account, please log in to renew your membership.';
	}

	return $errors;
}
add_filter( 'registration_errors', 'custom_registration_error_message', 10, 3 );