The Mastermind rogue's Master of Tactics feature (SCAG, p. 135; XGtE, p. 46) states:
Starting at 3rd level, you can use the Help action as a bonus action. Additionally, when you use the Help action to aid an ally in attacking a creature, the target of that attack can be within 30 feet of you, rather than within 5 feet of you, if the target can see or hear you.
Rules designer Jeremy Crawford clarified in a tweet that:
If you use the attack-aiding option in Help, the next ally who attacks the target gets the benefit.
Does this means that I can Help myself to give myself advantage on my next attack?
Best Answer
No
The Help action clearly states
Even if you read it separated from the second paragraph
Here, Crawford clearly states that "friendly creature" referes to someone else, not yourself, and even explains that, as in Inspiring Leader, if you could target yourself, it would state it explicitly.
Is a creature its own ally? Also probably no.
I would like to add another point: even if in 5E it might not be explicitly written, it is almost certainly intended that "an ally" is another creature (Note: As soon as I wrote that, I sanity-checked myself and got some doubt. Then I opened this question, so refer to it in case I'm wrong here). Check, for monsters from the Monster Manual, the feature
If the Wolf counted itself as an ally, this feature would be "the Wolf always has advantage on attack rolls", since it can only attack from melee (within 5ft.) and can only attack if he is not incapacitated. If this was intended, it would probably just say so.