In case anyone else encountered this problem. You have to edit your function and check the "has return value" box. Then your return will work. After this you have to use "Call Function" again and now you have a new field for saving results.
@marlo If you check append, previous file result won't be replaced, you probably have action, which doesn't have append checked. If you are 100% sure, that append is checked everywhere upload your project.
