The third-person and second-person feminine plurals end in nūn al-niswah. These forms are already indeclinable (mabnī), so lam does not remove their final nūn.
لَمْ يَفْعَلْنَ — those women did not do.
لَمْ تَفْعَلْنَ — you women did not do.
Contrast لَمْ يَفْعَلُوا, where the masculine-plural inflectional nūn is deleted.
Here the jussive is the grammatical status imposed by the particle; an indeclinable form can occupy that position without a visible change to its ending.
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Lam and the retained feminine-plural nūn
Negating the Imperfect with Lan and Lam: Subjunctive, Jussive and Full Paradigms
The third-person and second-person feminine plurals end in nūn al-niswah. These forms are already indeclinable (mabnī), so lam does not remove their final nūn.
لَمْ يَفْعَلْنَ — those women did not do.
لَمْ تَفْعَلْنَ — you women did not do.
Contrast لَمْ يَفْعَلُوا, where the masculine-plural inflectional nūn is deleted.
Here the jussive is the grammatical status imposed by the particle; an indeclinable form can occupy that position without a visible change to its ending.