With generic-negating lā, a singular ism that is neither muḍāf nor quasi-muḍāf is normally built on the sign it would take in the accusative; here rajula has fatḥ.
It is in the accusative position, but it is not inflected as رَجُلًا with tanwīn.
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Why Rajula Is Built on Fatḥ
Ism of Mā and Lā Resembling Laysa; Khabar of Lā of Generic Negation
With generic-negating lā, a singular ism that is neither muḍāf nor quasi-muḍāf is normally built on the sign it would take in the accusative; here rajula has fatḥ.
It is in the accusative position, but it is not inflected as رَجُلًا with tanwīn.