I get this message right after I finish filling in the database and admin information in the install process... as soon as I click "submit"... I see:

Unable to require /var/www/sites/mysupportnetonline.com/codoforum/sys//JsonSerializable.php

Can someone tell me what I can do to correct this issue?

Please use a new version of PHP, you have a very old version of PHP installed.

Thank @avinash . Your answer was wonderful.

Is there any guides around, that says more than just PHP and mysql, like what mods are needed etc. I am installing on a bare server, no cpanel.. so those details are helpful.

here's the error's i'm getting on the frontend, backend works fine so far..

I even moved the install to a default dir, from where I had it..
still the same, not sure why I thought that would help, oh yea bunch of /sys/xxxx.php errors, so checked permissions, and they were OK so I moved the install; yet as I said still the same or similar errors..

Unable to require /var/www/sys//DOMDocument.php
Fatal error: Uncaught Error: Class 'DOMDocument' not found in /var/www/sys/CODOF/Format.php:46 Stack trace: #0 /var/www/sys/CODOF/Forum/Topic.php(886): CODOF\Format::excerpt('<p>Hi, </p>\n<p...', 1, 'welcome-to-codo...' #1 /var/www/sys/Controller/Ajax/forum/topics.php(124): CODOF\Forum\Topic->gen_topic_arr_all_topics(Array) #2 /var/www/sys/Controller/forum.php(337): Controller\Ajax\forum\topics->get_topics(0, 'newest', false) #3 /var/www/routes.php(947): Controller\forum->topics(1) #4 /var/www/sys/Ext/limonade/limonade.php(429): {closure}() #5 /var/www/sys/CODOF/Access/Request.php(94): run() #6 /var/www/routes.php(964): CODOF\Access\Request::start() #7 /var/www/index.php(22): require('/var/www/routes...' #8 {main} thrown in /var/www/sys/CODOF/Format.php on line 46