Is pronouncing the "lām al-qamariyyah" (lunar "lām") in Al-Fatiha like the "lām al-shamsiyyah" (solar "lām") – such as saying "al-sirāt al-mustaqīm" as "aṣ-ṣirāṭ aṣ-ṣirāṭ" – considered a clear or a subtle mistake?

Omitting the "al" (the definite article) from the word "al-mustaqeem" (the straight) in Surat Al-Fatiha is considered a clear grammatical error (lahn jali) that invalidates the prayer. This is because Surat Al-Fatiha must be recited with all its letters and their vocalizations (tashdeedat), and omitting any letter from it prevents the recitation from being valid.