Notice (8) : session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/sessions) failed: Permission denied (13) [in /var/www/html/vendor/cakephp/cakephp/src/Http/Session.php, line 367]
Warning (2) : session_start(): Session cache limiter cannot be sent after headers have already been sent (output started at /var/www/html/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php:37) [in /var/www/html/vendor/cakephp/cakephp/src/Http/Session.php, line 367]
Warning (512) : Unable to emit headers. Headers sent in file=/var/www/html/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php line=37 [in /var/www/html/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 65]
Warning (2) : Cannot modify header information - headers already sent by (output started at /var/www/html/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php:37) [in /var/www/html/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 159]
Warning (2) : Cannot modify header information - headers already sent by (output started at /var/www/html/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php:37) [in /var/www/html/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 192]
Error: Could not start the session

Could not start the session 📋

Cake\Core\Exception\CakeException
CORE/src/Http/Session.php at line 368 (edit)
            return false;
        }
        if (!session_start()) {
            throw new CakeException('Could not start the session');
        }
        $this->_started true;

If you want to customize this error message, create templates/Error/error500.php