Psychohistory is pure fiction. The closest real equivalent is economics and economic predictions are essentially always wrong. I don't remember where exactly, but I recall reading that someone had done a mass study on economic forecasts and discovered that the only time they're correct was when they were predicting a continuation of whatever the current trend line was. Forecasting never managed to predict an actual change to the status quo, making such forecasts useless.

We can't even measure GDP accurately in quarters already passed, let alone predict what it will be in future, and surely predicting the movement of GDP is the most basic task a psychohistorian could possibly set themselves.