The output also shows a volume of zero; the pressure cannot be computed w/o a defined volume. The simulation pressure codes in CHARMM require the use of periodic boundary conditions.
You may be able to get a crude estimate of the pressure in an ad hoc way, before or after a simulation, using the PRESS INST command and supplying a volume and temperature. Estimating the volume of your droplet may be challenging. You may wish to research the topic of computing pressure in a droplet.
You could also try contacting the SSBP code authors.