First make sure you do not get that error when you manually use Insert File to insert a file into a container field.
Then you'll need to identify which step of your script is triggering this error code. Have you confirmed that this error occurs specifically on the Insert File Script step?
You may need to temporarily insert some show custom dialog or Pause/Resume steps into your script to see how far it gets before it encounters this error code.
Also, what version of FileMaker are you using and on what Operating System?
Oh, I got it. I was only trying to store a file reference to use later in an import step. I was trying to store this reference in a text field, but it still needed to be a container.