This message line is shown in the UserInterface's message window when running a script.
The line that is indicated in the message text is:
:set &hnd# = prep_process_file('win64', &filename#, , , 'UC_LOGON=LOGIN.GP')
The syntax of the script line is incorrect - there is one comma missing.
However, the error message U00020515 is not clear, can't tell if '&04' in the error message is intentional or if it is an unresolved placeholder?