Use the most recent version of CHARMM, it is free to download. C36 is very, very old.
Use the CHARMM 36 force field.
In this FF the phosphoserine patch is found in:
read rtf/para/coor/psf card name <filename>
instead of the super-obsolete, and error prone, series of commands: open, read, close
The patch command needs one (or more) SEGID and RESID to identify the residue that should be pathced; you have specified a residue name and number. The error message is quite clear.