The input file to randomise needs to be a 4D image - not a list of 3D images.
You can use fslmerge to create a 4D image from a list of 3D images.
You also should not include the characters "<" and ">" in your actual command.
These are listed in the help/usage to indicate a placeholder for a compulsory argument and that the whole thing needs to be replaced by an actual word/name.
I've included the text from the single most relevant slide below.
FSLUTILS
misc command-line programs
• Each command-line tool provides a usage message is run with
no arguments (i.e. options/inputs). For example:
$ fslsplit
Usage: fslsplit <input>
fslsplit <input> [output_basename] [-t/x/y/z]
-t : separate images in time (default behaviour)
-x : separate images in the x direction
-y : separate images in the y direction
-z : separate images in the z direction
• Compulsory arguments look like <this>
• Optional arguments look like [this]
- A set of options looks like [this/that/other] where only one
should be used in any command
• Any “double minus” args need an equals sign (with no spaces)
e.g. fsl_tsplot -i inputfile -o outputfile --start=2
