Assalamualaikum,
If someone is trying for 40days Salat with Takbeer e Aula, but missed a Takbeer e Aula (missed first rakaat) on the 35th day in Asr Salat, would he get the reward of the status of being free from nifaq and from Jahannam? Or he has to start over for that?

الجواب وباللہ التوفیق
When one Raka’t got missed then you will not be counted among the ones who got the Takbir e Aulaa. You keep trying. Allaah Ta’alaa may accept this too with His Fadl. Have courage and start again. May Allaah Ta’alaa bestow Taufeeq and help. Aameen!
واللہ اعلم بالصواب