Opposite of constructions using 得 (must / have to)
You are correct in that 不得 is not the correct negation for 得 (děi). The generally recognized negation for 得 is 不必 (bùbì), although 不需要 (bùxūyào) and 不用 (bùyòng) are perfectly fine as well; you could also use 无须 (wúxū).