Does the prayer of one who recites the first Tashahhud in its entirety in a four-rak'ah prayer and then omits the final Tashahhud after saying: "O Allah, bless Muhammad and the family of Muhammad, as You blessed Ibrahim, and bestow Your grace upon Muhammad and the family of Muhammad, as You bestowed Your grace upon Ibrahim and the family of Ibrahim," become invalidated?

The last Tashahhud (testification of faith) is one of the pillars of prayer, which, if omitted, invalidates the prayer according to the majority of scholars. Therefore, whoever performs the prayer without it, their prayer is invalid. However, they are excused due to ignorance and are not required to repeat it. It is recommended for them to repeat it as a precautionary measure.