The problem was the definition of terrorism.

There have been a number of UN resolutions against terrorism in general and against specific acts, such as hijackings and bombings, but a clear and agreed definition has been lacking.
(snip)
In the end, no definition was agreed though terrorism "in all its forms" is condemned.

And here's the Guardian's analysis:

Campaigners and diplomats who favoured a bold approach put much of the blame for the failure on John Bolton, the US ambassador to the UN, who introduced hundreds of late changes to the original document.