If you write the trigger without the unnecessary convoluted crappola, what happens?
```
CREATE TRIGGER InsertfehlerPruefung BEFORE INSERT ON Pruefung
BEGIN
SELECT RAISE(ABORT, 'not enough Fahrstundenminuten')
WHERE NEW.theorie == 0
AND (
SELECT SUM(dauer)
FROM fahrstunde
WHERE fahrstunde.schueleremail == NEW.email
) >= 180;
END;
```
I don't speak whatever language those variables are in but I suspect that your >= comparison is wrong-about.
I therefore suspect that your whole logic is broken and I would only comment that convoluted thinking and complication for complication sake does not usually result in what was intended. In other words, embrace the KISS principle!