Hello,
For now, hard to tell as you have a lots of different error and with different nature and root.
First of all, use your Joomla error reporting to simple, as shown on my screenshot :
That's will allow to see among all this error (some of them are minor... to get the real checkout root issue.
Plus note that some of your error are
Joomla related, like by example this one :
"[13-Dec-2017 09:10:50 UTC] PHP Warning: session_start(): Failed to read session data: user (path: /tmp) in /home/vrayntqp5bqe/public_html/libraries/joomla/session/handler/native.php on line 260
[13-Dec-2017 09:12:38 UTC] PHP Warning: session_start(): Failed to read session data: user (path: /tmp) in /home/vrayntqp5bqe/public_html/libraries/joomla/session/handler/native.php on line 260"
Awaiting your returns to move forward.
Regards