I have a custom library with documents and workflow for approval process. The problem is I cannot extract assigned to field from task list to library's column.
I'm trying to set field to current item but it doesn't work
I think the problem is your "find the list item". Every task which is created in the workflow will create a variable at the end of the line. So you have to match the "Field: ID" with the "Value: variable ...". It should be written in (). Here is a link to a picture which shows you how to match ID and variable. This should help you.
Create a field in your library in my example called "gebucht von" and the task action is "collect data from user". Just to show you how to handle it.
Add in your workflow the list action "set field in current item". Add the field in which you want to use the informations of the task list. Should looks so:
Set "gebucht von" (library field) to "task list:asssigned to"
To get the "task list:assigned to" you have to choose so:
Above id reference for how I am using. But in "Find the List Item" just to show you how you can try.