Why does it give an error when installing plugins?

Question

WordPress

The site has the WC Ukraine Shipping New Mail plugin installed, and when you try to install Ukrposhta, Justin for WooCommerce gives an error

Warning: The magic method morkva\JustinShip\classes\JustinShip::__wakeup() must have public visibility in /wp-content/plugins/justin/classes/JustinShip.php on line 28

Warning: The magic method deliveryplugin\Ukrposhta\classes\UkrposhtaShipping::__wakeup() must have public visibility in /wp-content/plugins/woo-ukrposhta/classes/ukrposhtaShipping.php on line 26

The plugin generated 232 characters of unexpected output on activation. If you encounter a “headers already sent” error with RSS feeds or other problems, then try deactivating or removing this plugin.

What could be the reason?