From the error it seems, you have not assigned the proper directory+file name to file transfer activity. the error indicates that there is no file name so script considers folder name as file and fails.
you need to ensure that directory does exist and the existence of the file is optional.
The 'Server folder' contains the file path to the file on the sFTP that you wish to import, you will be provided the filename in the ticket. Simply paste in the filename provided after the folder directory (with extension) after '/custom-folder/file-name.csv'