The rules:
Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.
You don't need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn't incapacitated, and you don't have disadvantage on the attack roll. (PHB, pg 96)
and
In addition, you don't need advantage on your attack roll to use your Sneak Attack if no creature other than your target is within 5 feet of you. All the other rules for the Sneak Attack class feature still apply to you. (SCAG, pg 136)
This rule has been errata'd and will be changed in the upcoming Xanathar's Guide To Everything.
The way I understand them:
Step 1. identify the rules in the swashbuckler's feature
- if your target is the only creature within 5' of you then you do not need advantage
All the other rules for the Sneak Attack class feature still apply to you
Step 2 identify "all other" rules
if your target is the only creature within 5' of you then you do not need advantage
if you have advantage [OR] you don't need advantage then you can deal an extra 1d6 damage once per turn
the attack must use a finesse or a ranged weapon
if your target has a non-incapacitated enemy that is not you within 5' of it [AND] you don't have disadvantage then you don't need advantage on the attack roll
Given: no creature other than your target is within 5 of you
Simplified: using standard logical operations
you don't need advantage on your attack roll
you can deal an extra 1d6 damage once per turn
the attack must use a finesse or a ranged weapon
Am I correct, a swashbuckler can sneak attack with disadvantage?
If not, please identify and explain the error?
NOTE:
- if your target has a non-incapacitated enemy that is not you within 5' of it [AND] you don't have disadvantage then you don't need advantage on the attack roll
must be applied if and only if the non-incapacitated enemy that is not you is within 5' of your target and within 5' of you, which is typically an easy situation to fix.