PHP error: Uncaught Error: Call to undefined function

Hello, I was using this plugin successfully with my previous WordPress site running on my own server: Nginx with PHP 5.6 (I think from memory) on Rasbian/Debian Wheezy. I rebuilt the server with Rasberry OS/Debian Buster and PHP 7.4.

Wordpress is working fine apart from this plugin. I recreated the SSH user and keys and believe these are okay as I can login outside of PHP with them.

The NGINX log shows:

2021/03/02 06:42:39 [error] 556#556: *15111 open() "/mnt/webfiles/websites/cabincontrol.tlcb.uk404.html" failed (2: No such file or directory), client: 10.5.248.122, server: cabincontrol.tlcb.uk, request: "GET /favicon.ico HTTP/1.1", host: "cabincontrol.tlcb.uk", referrer: "https://cabincontrol.tlcb.uk/heating.html"
2021/03/02 08:36:49 [error] 556#556: *35635 FastCGI sent in stderr: "PHP message: PHP Fatal error:  Uncaught Error: Call to undefined function xml_parser_create() in /mnt/webfiles/websites/79th.tlcb.uk/wp-content/plugins/ssh-sftp-updater-support/phpseclib/Crypt/RSA.php:1468
Stack trace:
#0 /mnt/webfiles/websites/79th.tlcb.uk/wp-content/plugins/ssh-sftp-updater-support/phpseclib/Crypt/RSA.php(1785): Crypt_RSA->_parseKey()
#1 /mnt/webfiles/websites/79th.tlcb.uk/wp-content/plugins/ssh-sftp-updater-support/class-wp-filesystem-ssh2.php(97): Crypt_RSA->loadKey()
#2 /mnt/webfiles/websites/79th.tlcb.uk/wp-admin/includes/file.php(1861): WP_Filesystem_SSH2->connect()
#3 /mnt/webfiles/websites/79th.tlcb.uk/wp-admin/includes/class-wp-upgrader.php(193): WP_Filesystem()
#4 /mnt/webfiles/websites/79th.tlcb.uk/wp-admin/includes/class-plugin-upgrader.php(293): WP_Upgrader->fs_connect()
#5 /mnt/webfiles/websites/79th.tlcb.uk/wp-admin/includes/ajax-actions.php(4452): Plugin_Upgrader->bulk_upgrade()
#6 /mnt/webfiles/websites/79th.tlcb.uk/wp-includes/class-wp-hook.php(287): wp_ajax_upd" while reading response header from upstream, client: 10.5.248.124, server: 79th.tlcb.uk, request: "POST /wp-admin/admin-ajax.php HTTP/1.1", upstream: "fastcgi://unix:/var/run/php/php7.4-fpm.sock:", host: "79th.tlcb.uk", referrer: "https://79th.tlcb.uk/wp-admin/plugins.php"

I’m sure it’s a dependency I have not installed. I installed:
php7.4-ssh2
and is at version: 1.2+0.13-9+0~20210223.17+debian10~1.gbp17d23d

and
php-phpseclib
which is at version: 2.0.14-1

What am I missing?

thanks