Your file names are 3 levels deep, the other datasets are 2 at most. Try adding a List.Flatten node between the file names and the inputs and see if that solves the issue, or utilize List At Level function to ensure all items process at level 2.
Thanks… I seem to have got it working now but not without further errors along the way. I’ll study the differences between the first graph and the final graph to see what the issues were.