gistfile1.txt

<?php

add_filter( 'mepr-validate-login', 'secupress_support_11052021_no_admins' );
function secupress_support_11052021_no_admins( $errors ) {
	if ( ! isset( $_SERVER['REQUEST_METHOD'], $_POST['log'] ) || 'POST' !== $_SERVER['REQUEST_METHOD'] ) {
		return $errors;
	}

	// Check for login by email address
	$by   = is_email( $_POST['log'] ) ? 'email' : 'login'; 
	$user = get_user_by( $by, $_POST['log'] );

	if ( ! is_wp_error( $user ) && user_can( $user, 'administrator' ) ) {
		$errors[] = 'Administrators can not login via this page.';
	}

	return $errors;
	
}