The PIV is not in the mechanical room, it's outside where it is supposed to be. The addressable module for the PIV is located in the mechanical room and labeled that way so it can be found when it needs service.
An addressable panel, like the Siemens panel you're working on, uses a Signaling Line Circuit (SLC) for communication. This is really a computer data and power buss. This circuit isn't connected to any device like the pull stations, waterflow switches, gatevalve tamper switches, it's connected to intermediary modules (usually just called modules), and then these modules are connected to the switches using Class B circuitry.
The module is basically monitoring the PIV. It looks like the programmer of the panel didn't "miss program" the panel. Rather, the programmer is trying to help you, the servicer of the system, find the module to fix the system.
"PIV" is the device that is being monitored. Yes, it's the correct "Post Indicator Valve" with a knob to turn off the sprinkler water outside the building. "Mechanical Room", on the other hand, is where you can find the module. That is placed in the message showing on the panel so you don't have to hunt all over the building looking for the module.
"Error" is some sort of trouble signal. It is saying that this module is having trouble, is completely broken, or that the Class B wiring to the PIV is having trouble. Your job is to find that module, which is where you have to start to troubleshoot. This is the device that the panel is really connected to, and not the PIV.
Keep in mind that the module has two different sides, the SLC side and the Class B side.
Using your voltmeter, the SLC side should measure an unsteady (constantly changing) voltage between 10 volts and 24 volts.
The Class B side should show a normal Class B voltage (you'll have to figure that out because each manufacturer uses a different voltage to be "normal").
The module might be having a problem, the wiring out to the PIV might be having a problem, or the end-of-line resistor at the PIV might be having a problem. Replace the module if that is what is having a problem, fix the wiring to the PIV if that is what is having a problem, or replace the end-of-line resistor if that is what is having a problem.