Why am I getting this error?
This issue is referenced in Dkan developpers Google group.

You probably have updated Feeds modules in Dkan profiles folder? Read the above post carefully and consider trying proposed solution :


Keep the Dkan profile original code in place
Download Feeds 7.x-2.0-beta4 to sites/all/modules/contrib
Rebuild registry 


EDIT : I browsed both Feeds and Dkan feeds.module history. I couldn't find any implementation of feeds_node_prepare() in Feeds, while Dkan contrib feeds.module does implement it. 

Seems that Feeds in DKAN profile contrib modules directory is a fork, not official release.