If you get "Content Error" when importing to Content, K2, ZOO, ... adapters using Get Fulltext processor, you are not doing anything wrong.
The first thing you can do is go to your Pipe, change "Get Fulltext" processor setting > Get cURL value from "Level 1" to "Level 2", and test again. If it still doesn't owrk, it is probably that processor doesn't work with your source.
Please take a look at this: My pipe doesn't work, I can't grab full text from RSS source