The following should also work (note that there is no need to have OPEN and CLOSE commands with READ and WRITE commands):
read psf card name step3_pbcsetup.psf
read coor pdb name "PFF_8cell_last.pdb"
open unit 21 read unform name "PFF_8cell_10ns.dcd"
Using double quotes preserves case in filenames when you read files.