Please describe in detail how you do it and when it fails to load a save from a file.
Hmm, it's not very clear, is the “Import file” button not active (that's strange since it has no conditions to be inactive...)? Or is the browser/game not opening the file selection dialog after you clicking "Import File"? Or some another problem after you selected imported .json file in file chooser dialog?
Just in case, a few points: The lower sub-window of saving and loading via file is needed only for transferring saves to other versions of the game (for example, from the Itch.io game window to a Standalone version). For all other saves it is better to use the main Load/Save window, there - first you need to select a slot from which you want to load, without this the Load button will not be highlighted and will not be active (Maybe that confused you?).