Ever had this nice error:

I first got this error when I was migrating the workflow from development to live. I just copied the workflow and linked the list and items correctly. When I press finish it saves the changes but cannot associate the workflow with the list. The problem was because it was the first workflow on the site, there was no folder created for the workflows. A copy paste, copied the workflow in the root directory. Look for the difference in the pictures below!

This doesn't work:

This works:

In both pictures you will find a workflow folder, this was because I had taken the screen after the folder was created. But I dragged the workflow to the root and the same problem arrose. So the solution is to put the workflow in the correct workflow folder. If it does not exists yet:create a dummy workflow and the system will create the workflow folder for you. Then drag your workflow in this folder and now it can associate the workflow with the list. Remove the dummy workflow.