DND_Reborn
The High Aldwin
So, the idea was basically this (as an example):
What the errata means is that, instead of using a weapon in a melee weapon attack, you can replace it with an unarmed strike (which means it still is a melee weapon attack).
If an ability states 'an attack with a melee weapon', then it requires a weapon.
Its confusing and dumb, but there's the rules.
Since an unarmed strike counts as a melee weapon attack, you could use Divine Smite on it. However, since it does not count as a weapon, you cannot use Sacred Weapon on it.
Wow, let's just make things needlessly confusing.