How to solve this?

I need to develop a decent template for my website, so I tried to upload joomla_nicepage_plugin-3.26.0.zip to my Joomla 4 (non-beta, but real Joomla 4) site. No way... Troubles.
I activated the debugger and saw :

********** FATAL ERROR ********************************

HTTP 500 Whoops, looks like something went wrong.

Error: Class NicepageInstallerJAdministratorWrapper contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Joomla\Application\AbstractApplication::doExecute)

************************************ EXCEPTION tab **********************************

in /home/mywebsite/public_html/tmp/install_614d90e1594ce/install.script.php (line 414)

    if (!class_exists('NicepageInstallerJAdministratorWrapper')) {
        /**
         * Class NicepageInstallerJAdministratorWrapper
         */
        class NicepageInstallerJAdministratorWrapper extends JApplicationCms
        {
            /**
             * @var JApplicationCms
             */
            protected $app;

************************************ STACK Trace tab **********************************

Symfony\Component\ErrorHandler\Error\FatalError:
Error: Class NicepageInstallerJAdministratorWrapper contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Joomla\Application\AbstractApplication::doExecute)

at /home/mywebsite/public_html/tmp/install_614d8a29b8f81/install.script.php:414