bug, please help

[quantrpeter]

PHP Fatal error: Uncaught Error: Call to undefined method GuzzleHttp\Utils::chooseHandler() in /home/quantr.hk/www/wordpress/wp-content/plugins/quantr-eshop/public/vendor/guzzlehttp/guzzle/src/functions.php:61\nStack trace:\n#0 /home/quantr.hk/www/wordpress/wp-content/plugins/backwpup/vendor/guzzlehttp/guzzle/src/HandlerStack.php(42): GuzzleHttp\choose_handler()\n#1 /home/quantr.hk/www/wordpress/wp-content/plugins/backwpup/vendor/guzzlehttp/guzzle/src/Client.php(65): GuzzleHttp\HandlerStack::create()\n#2 /home/quantr.hk/www/wordpress/wp-content/plugins/quantr-eshop/public/vendor/league/oauth2-client/src/Provider/AbstractProvider.php(132): GuzzleHttp\Client->__construct(Array)\n#3 /home/quantr.hk/www/wordpress/wp-content/plugins/quantr-eshop/public/vendor/league/oauth2-client/src/Provider/GenericProvider.php(99): League\OAuth2\Client\Provider\AbstractProvider->__construct(Array, Array)\n#4 /home/quantr.hk/www/wordpress/wp-content/plugins/quantr-eshop/public/login.php(36): League\OAuth2\Client\Provider\GenericProvider->__construct(Ar in /home/quantr.hk/www/wordpress/wp-content/plugins/quantr-eshop/public/vendor/guzzlehttp/guzzle/src/functions.php on line 61, referer: https://www.quantr.hk/shop/?type=siteBuilder

php --version
PHP 7.3.29-1+ubuntu18.04.1+deb.sury.org+1 (cli) (built: Jul 1 2021 16:08:04) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.29, Copyright (c) 1998-2018 Zend Technologies
with Zend OPcache v7.3.29-1+ubuntu18.04.1+deb.sury.org+1, Copyright (c) 1999-2018, by Zend Technologies