If your entire run is in Zone 5, likely your Critical Power was not set right at the time of your run. Your RSS will be zero as well.
If you go for a run and see that your run stress score (RSS) is 0, and your entire run was in power zone 5, you need to recalculate your critical power. For whatever reason, it reset and was not recorded correctly. Once you have recalculated your critical power (either with the Stryd app on your phone, or PowerCenter), you can recalculate your run: hover over the activity in the calendar in PowerCenter, and you will get several options. Click on the circular arrow to recalculate the RSS.
Note that you need to refresh your browser window to see the update RSS values. Graphs on the Improve tab might only update after you recorded a new activity.
If need additional assistance, contact firstname.lastname@example.org.