I found 500 error on saveOrder function in file “shoppingcart.php” on line 125.
When i click checkout button Ajax Post to /save_order/task:pay
I test on localhost and AWS Ubuntu have same result. Screen Shot 2559-05-03 at 11
Replied in Gitter, for the future 
was using a currently officially unreleased plugin that required ShoppingCart 1.1.x, but had 1.0.x installed