Warning (2): session_start(): open(/var/cpanel/php/sessions/nf-php74/sess_pcueq8elf8sq922g3b95h4cuno, O_RDWR) failed: No space left on device (28) [CORE/src/Http/Session.php, line 355]
Warning (2): session_start() [<a href='https://secure.php.net/function.session-start'>function.session-start</a>]: Failed to read session data: files (path: /var/cpanel/php/sessions/nf-php74) [CORE/src/Http/Session.php, line 355]
Warning (512): Unable to emit headers. Headers sent in file=/home/bestbrand25/public_html/vendor/cakephp/cakephp/src/Error/Debugger.php line=831 [CORE/src/Http/ResponseEmitter.php, line 72]
Warning (2): Cannot modify header information - headers already sent by (output started at /home/bestbrand25/public_html/vendor/cakephp/cakephp/src/Error/Debugger.php:831) [CORE/src/Http/ResponseEmitter.php, line 169]
Warning (2): Cannot modify header information - headers already sent by (output started at /home/bestbrand25/public_html/vendor/cakephp/cakephp/src/Error/Debugger.php:831) [CORE/src/Http/ResponseEmitter.php, line 198]
Error: Could not start the session

Could not start the session

RuntimeException
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/home/bestbrand25/public_html/vendor/cakephp/cakephp/src/Http/Session.php Toggle Arguments
            return false;
        }
        if (!session_start()) {
            throw new RuntimeException('Could not start the session');
        }
        $this->_started true;

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