Since PHP 8.0 the xmlrpc extension is not shipped with php any more.
Go to https://pecl.php.net/package/xmlrpc and download the latest version.
For linux you might use something like that:
$ wget https://pecl.php.net/get/xmlrpc-1.0.0RC3.tg
$ tar xfv xmlrpc-1.0.0RC3.tg
$ cd xmlrpc-1.0.0RC3
$ phpize && ./configure && make && make test && make install
Add the build extension in your php.ini section and don't forget to restart php-fpm or your webserver after the installation.
To verify the installation, you can use something like this (should at least return the line "xmlrpc"):
$ php -i | grep xmlrpc | grep -v "xmlrpc_error"