There were three areas of failure at the G20 in Hamburg.
The most obvious and easy to identify was Donald man-child Trump.
I'll let this Australian reporter sum it up better than I could.

The Saker has a very good article on the upcoming G20 conference at which Trump and Putin are scheduled for talks on the sidelines. In it he outlines the extremely low possibility that there will be meaningful discussions, mostly due to the intransigence of those that have taken complete control of the Executive Branch of the US government.