When affected by the Slow spell:
An affected target’s speed is halved, it takes a -2 penalty to AC and Dexterity saving throws, and it can’t use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature’s abilities or magic items, it can’t make more than one melee or ranged attack during its turn.
A fighter is affected by Slow. Can they use Action Surge for two actions (attacks) in a turn?
Action Surge – Starting at 2nd level, you can push yourself beyond your normal limits for a moment. On your turn, you can take one additional action.
Edited for clarity and expanding upon a comment.
The text of the Slow spell is unclear. Of course there is a literal reading, but that is complicated by the principle of "Specific beats General".
The Slow spell refers to Action or Bonus action in the singular. Normally each turn creatures get one action and one bonus action. Slow limits this to choosing just one. Many creatures have multiattack which Slow limits to a single attack. In general there is just one action. The specific feature Action Surge grants an extra action and it seems reasonable that an Action Surging fighter gets a second attack (but not a multiattack) with the extra action.
Is this a reasonable interpretation? If not, why would the general Slow spell beat the specific Action Surge?
Best Answer
Simply put, No
The quoted text explains very clearly what happens as per the description of Slow. It states that a creature affected by slow can not make more than a melee or ranged attack that turn. This is regardless of abilities or magic items the creature has that could grant it additional attacks (e.g. Action Surge or Scimitar of Speed).
So a Fighter could Action Surge. However, because of the rules of the spell, they could not use the Attack Action to make an extra attack since it is still inside their turn and they (assumedly) have already made an attack that round.
The only way for the Action Surge to give an attack for the attack action would be if the fighter at hand used their normal action for the Dash, Disengage, Dodge, or Hide action.