SendGrid API

Your software
My Mautic version is: v2.16.3
My PHP version is: 7.2.24-0ubuntu0.18.04.6
My Database type and version is: mysqlnd 5.0.12-dev - 20150407

Your problem
My problem is: I see this error from on my vps server and my follow up messages are not being sent by Mautic. Does anyone know how to solve this?

These errors are showing in the log:
[2020-11-04 14:02:24] mautic.NOTICE: PHP Notice - SessionHandler::gc(): ps_files_cleanup_dir: opendir(/var/lib/php/sessions) failed: Permission denied (13) - in file /var/www/html/vendor/symfony/http-foundation/Session/Storage/Proxy/SessionHandlerProxy.php - at line 86

Steps I have tried to fix the problem: