This error means that you have a wrong action (hook). meaning the code is trying to call a function that does not exists. But, from the path the error is coming from, I think: a) you may have tried to change the core code b) or more likley that your WordPress install is not complete, and you are missing files.

This error means that you have a wrong action (hook). meaning the code is trying to call a function that does not exists. But, from the path the error is coming from, I think:

1. you may have tried to change the core code
   
2. or more likley that your WordPress install is not complete, and you are missing files.