Repair Failed Joomla 3 to 4 Migration

During Migration our site failed and is now showing errors (detailed below)

This project is to get the site working with Joomla 4 with no loss to the data on the site currently, we do have available on our server a JPA back up file of it on version 3.

We are aware once working on Joomla 4, we have to update our template which is Joomlart Teline V, we have the latest version ready to deploy.

This is a time sensitve project which could lead to further works. Must be able to complete within 24 hours of accepting project.

you can see the errors below or here: www.brca.org/administrator


Errors:

Symfony\Component\ErrorHandler\Error\
UndefinedMethodError
in /home/brca/public_html/plugins/system/jsn_system/jsn_system.php (line 339)
} public function onBeforeCompileHead() { require_once(JPATH_ADMINISTRATOR.'/components/com_jsn/defines.php'); $xml = JFactory::getXML(JPATH_ADMINISTRATOR .'/components/com_jsn/jsn.xml'); $version = (string)$xml->version; $app=JFactory::getApplication(); // Load Config
plgSystemJsn_System->onBeforeCompileHead()
in /home/brca/public_html/libraries/src/Plugin/CMSPlugin.php (line 279)
in /home/brca/public_html/libraries/vendor/joomla/event/src/Dispatcher.php -> Joomla\CMS\Plugin\{closure} (line 486)
Dispatcher->dispatch('onBeforeCompileHead', object(Event))
in /home/brca/public_html/libraries/src/Application/EventAware.php (line 107)
WebApplication->triggerEvent('onBeforeCompileHead')
in /home/brca/public_html/libraries/src/Document/Renderer/Html/MetasRenderer.php (line 61)
MetasRenderer->render(null, array(), null)
in /home/brca/public_html/libraries/src/Document/HtmlDocument.php (line 565)
HtmlDocument->getBuffer('metas', null, array())
in /home/brca/public_html/libraries/src/Document/HtmlDocument.php (line 848)
HtmlDocument->_renderTemplate()
in /home/brca/public_html/libraries/src/Document/HtmlDocument.php (line 633)
HtmlDocument->render(false, array('template' => 'atum', 'directory' => '/home/brca/public_html/administrator/templates', 'debug' => '1', 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
in /home/brca/public_html/libraries/src/Document/ErrorDocument.php (line 139)
ErrorDocument->render(false, array('template' => 'atum', 'directory' => '/home/brca/public_html/administrator/templates', 'debug' => '1', 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
in /home/brca/public_html/libraries/src/Error/Renderer/HtmlRenderer.php (line 78)
HtmlRenderer->render(object(Error))
in /home/brca/public_html/libraries/src/Exception/ExceptionHandler.php (line 121)
ExceptionHandler::render(object(Error))
in /home/brca/public_html/libraries/src/Exception/ExceptionHandler.php (line 72)
ExceptionHandler::handleException(object(Error))
in /home/brca/public_html/libraries/src/Application/CMSApplication.php (line 323)
CMSApplication->execute()
in /home/brca/public_html/administrator/includes/app.php (line 61)
require_once('/home/brca/public_html/administrator/includes/app.php')
in /home/brca/public_html/administrator/index.php (line 32)