As for leaving off the tashdeed in the letter (ض) in the word (وَلَا الضَّالِّينَ) in al-Fatiha, then some of the Scholars say that the Fatiha does not count.
Shaikh Ibn ul-‘Uthaymeen (may Allah have mercy upon him) said: “His (the author’s) statement: “Or leaves off tashdeed” that is: leaves tashdeed in a letter from al-Fatiha and he recites it unstressed , such as reciting the letter (الباء) unstressed in His statement:
All the praises and thanks be to Allah, the Lord of the ‘Alamin (mankind, jinns and all that exists) [1:1]
Then it would not be valid. It is not valid because a letter with a tashdeed is actually composed of two letters. If a person leaves off the tashdeed, he has removed a letter” [Sharh al-Mumti’ (3/63)]
Shaikh al-Fawzan (may Allah preserve him) said: “. . . and verily his prayer is invalid if he made a major error which changes the meaning of the recitation of al-Fatiha, or leaves off a tashdeed (stress) or a letter…” End quote. [Reference]
Similar speech has come from Imam an-Nawawi (may Allah have mercy upon him) in al-Majmoo’ (3/347), Imam Ibn Qudamah (may Allah have mercy upon him) in al-Mughni (2/154), Shaikh Muhammad bin Ibraahim Ala Shaikh (may Allah have mercy upon him) in Sharh Adaab Mashi ila as-Salah (pg. 31), and others.
What about if someone is ignorant regarding this issue?
Shaikh Ibn ul-‘Uthaymeen (may Allah have mercy upon him) was asked:
If the one praying recited al-Fatiha without tashdeed, is his prayer invalid?
“If he recited:
(الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ (1) الرَّحْمَنِ الرَّحِيمِ (2) مَالِكِ يَوْمِ الدِّينِ (3
All the praises and thanks be to Allah, the Lord of the ‘Alamin (mankind, jinns and all that exists). The Most Beneficent, the Most Merciful. The Only Owner (and the Only Ruling Judge) of the Day of Recompense (i.e. the Day of Resurrection) [1:1-3]
And he does not make tashdeed on the Baa and on the Daal, then it is not valid even if it was done out of ignorance. However, if he is ignorant, and a long time has past [i.e. after he recited it in this manner], and he is from those who do not know these things, then there is nothing upon him” [Ta’leeqaat ‘alal-Kaafi Li Ibn Qudamah (1/375)]
Thus a person should be careful in reciting the Fatiha in the prayer and learn how to recite it properly.
And Allah knows best
Faisal bin Abdul Qaadir bin Hassan