Why is my Psychology tab empty?

Because there are no debriefs with scores behind it. The app asks for at least two debrief sessions with scores filled in.

With none at all, the tab says so plainly — "No debrief sessions yet" — rather than showing empty charts.

The threshold is low on purpose: two debrief sessions with their scores completed is enough to unlock the optimal-state profile, tilt detection and decision-quality analysis. It is a correlation between your state and your results, and two points is the minimum that can produce one at all — though of course it improves quickly with more.

If the tab is empty and you have been doing debriefs, the usual cause is that the debriefs exist but the mental scores inside them were left blank. The scores are what these charts correlate; a written reflection with no numbers gives them nothing to plot.