● Earlier present-specific form: لَيَكْتُبُ — lām at the beginning, no emphatic nūn at the end.
● Emphatic form here: لَيَكْتُبَنَّ — emphatic lām at the beginning and heavy nūn at the end.
● Separate particle: لَنْ — written separately and not this construction.
Read the whole form; the first letter alone is not enough.