Why is my RSS value 0 in PowerCenter?

PowerCenter calculates various metrics and training plans based on your critical power. Critical Power (threshold power) represents the highest power that a runner can maintain in a quasi-steady state without fatiguing, where the duration may range from 30-70 minutes, depending on the individual. You can read more about how to measure your CP here:

How do I perform a critical power test and get my critical power and zones?

If critical power is not is set in PowerCenter (i.e. 0), your RSS values will be 0 since power is critical to determine your RSS. RSS is a single number to help runners understand their day-by-day training. It takes into account the volume and intensity of your training session to give you a single number, or “score.”