You have to replace the "/"s with "-"s

try
with Kix 4.61
 Code:
 Move "H:\Backup\backup.bkf" "H:\Backup\backup " + replace(@date,"/","-") + ".bkf"


older kix
 Code:
 Move "H:\Backup\backup.bkf" "H:\Backup\backup " + join(split(@date,"/"),"-") + ".bkf"