لَا يَدْخُلُ الْجَنَّةَ مَنْ لَا يَأْمَنُ جَارُهُ بَوَائِقَهُ contains two occurrences of لَا:
The first, in لَا يَدْخُلُ, negates entering.
The second, in لَا يَأْمَنُ, negates the neighbor’s safety.
مَنْ functions like a relative pronoun: “the person whose neighbor…”. The following clause identifies the person receiving the warning.
The practical implication is that worship should be accompanied by care for the neighbor’s safety, property, dignity, and peace.