Is it necessary to roll the "R" sound when pronouncing it in Al-Fatiha, and does not rolling it affect the validity of the recitation?

The letter 'Raa' has the attribute of 'takrir' (repetition), and one must be careful not to overdo it. The correct pronunciation is for the tip of the tongue to vibrate gently just once when emitting the sound. Its repetition must be concealed, and this is achieved by the tip of the tongue touching the base of the front teeth, to prevent the continuation of the repetition. This attribute is more apparent in the 'Raa' with a shaddah (doubled 'Raa'). Overthinking the articulation points of letters (makharij al-huruf) is one of the Devil's stratagems, and one must be wary of indulging in it.